#include <bits/stdc++.h>
using namespace std;
const long long INF = 1LL << 60;
const long double EPS = 1e-10;
string operator*(const string& s, int k) {
  if (k == 0) return "";
  string p = (s + s) * (k / 2);
  if (k % 2 == 1) p += s;
  return p;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
struct Edge {
  long long to, cap, rev;
  Edge(long long _to, long long _cap, long long _rev) {
    to = _to;
    cap = _cap;
    rev = _rev;
  }
};
void add_edge(vector<vector<Edge>>& G, long long from, long long to,
              long long cap, bool revFlag, long long revCap) {
  G[from].push_back(Edge(to, cap, (long long)G[to].size()));
  if (revFlag)
    G[to].push_back(Edge(from, revCap, (long long)G[from].size() - 1));
}
long long max_flow_dfs(vector<vector<Edge>>& G, long long v, long long t,
                       long long f, vector<bool>& used) {
  if (v == t) return f;
  used[v] = true;
  for (int i = 0; i < G[v].size(); ++i) {
    Edge& e = G[v][i];
    if (!used[e.to] && e.cap > 0) {
      long long d = max_flow_dfs(G, e.to, t, min(f, e.cap), used);
      if (d > 0) {
        e.cap -= d;
        G[e.to][e.rev].cap += d;
        return d;
      }
    }
  }
  return 0;
}
long long max_flow(vector<vector<Edge>>& G, long long s, long long t) {
  long long flow = 0;
  for (;;) {
    vector<bool> used(G.size());
    for (int(i) = (int)(0); i < (int)(used.size()); i++) used[i] = false;
    long long f = max_flow_dfs(G, s, t, INF, used);
    if (f == 0) {
      return flow;
    }
    flow += f;
  }
}
void BellmanFord(vector<vector<Edge>>& G, long long s, vector<long long>& d,
                 vector<long long>& negative) {
  d.resize(G.size());
  negative.resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) d[i] = INF;
  for (int(i) = (int)(0); i < (int)(d.size()); i++) negative[i] = false;
  d[s] = 0;
  for (int(k) = (int)(0); k < (int)(G.size() - 1); k++) {
    for (int(i) = (int)(0); i < (int)(G.size()); i++) {
      for (int(j) = (int)(0); j < (int)(G[i].size()); j++) {
        if (d[i] != INF && d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
        }
      }
    }
  }
  for (int(k) = (int)(0); k < (int)(G.size() - 1); k++) {
    for (int(i) = (int)(0); i < (int)(G.size()); i++) {
      for (int(j) = (int)(0); j < (int)(G[i].size()); j++) {
        if (d[i] != INF && d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
          negative[G[i][j].to] = true;
        }
        if (negative[i] == true) negative[G[i][j].to] = true;
      }
    }
  }
}
void Dijkstra(vector<vector<Edge>>& G, long long s, vector<long long>& d) {
  d.resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) d[i] = INF;
  d[s] = 0;
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      q;
  q.push(make_pair(0, s));
  while (!q.empty()) {
    pair<long long, long long> a = q.top();
    q.pop();
    if (d[a.second] < a.first) continue;
    for (int(i) = (int)(0); i < (int)(G[a.second].size()); i++) {
      Edge e = G[a.second][i];
      if (d[e.to] > d[a.second] + e.cap) {
        d[e.to] = d[a.second] + e.cap;
        q.push(make_pair(d[e.to], e.to));
      }
    }
  }
}
void WarshallFloyd(vector<vector<Edge>>& G, vector<vector<long long>>& d) {
  d.resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) d[i].resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) {
    for (int(j) = (int)(0); j < (int)(d[i].size()); j++) {
      d[i][j] = ((i != j) ? INF : 0);
    }
  }
  for (int(i) = (int)(0); i < (int)(G.size()); i++) {
    for (int(j) = (int)(0); j < (int)(G[i].size()); j++) {
      chmin(d[i][G[i][j].to], G[i][j].cap);
    }
  }
  for (int(i) = (int)(0); i < (int)(G.size()); i++) {
    for (int(j) = (int)(0); j < (int)(G.size()); j++) {
      for (int(k) = (int)(0); k < (int)(G.size()); k++) {
        chmin(d[j][k], d[j][i] + d[i][k]);
      }
    }
  }
}
bool tsort(vector<vector<Edge>>& graph, vector<int>& order) {
  int n = graph.size(), k = 0;
  vector<long long> in(n);
  for (auto& es : graph)
    for (auto& e : es) in[e.to]++;
  priority_queue<long long, vector<long long>, greater<long long>> que;
  for (int(i) = (int)(0); i < (int)(n); i++)
    if (in[i] == 0) que.push(i);
  while (que.size()) {
    int v = que.top();
    que.pop();
    order.push_back(v);
    for (auto& e : graph[v])
      if (--in[e.to] == 0) que.push(e.to);
  }
  if (order.size() != n)
    return false;
  else
    return true;
}
class lca {
 public:
  const int n = 0;
  const int log2_n = 0;
  std::vector<std::vector<int>> parent;
  std::vector<int> depth;
  lca() {}
  lca(const vector<vector<Edge>>& g, int root)
      : n(g.size()),
        log2_n(log2(n) + 1),
        parent(log2_n, std::vector<int>(n)),
        depth(n) {
    dfs(g, root, -1, 0);
    for (int k = 0; k + 1 < log2_n; k++) {
      for (int v = 0; v < (int)g.size(); v++) {
        if (parent[k][v] < 0)
          parent[k + 1][v] = -1;
        else
          parent[k + 1][v] = parent[k][parent[k][v]];
      }
    }
  }
  void dfs(const vector<vector<Edge>>& g, int v, int p, int d) {
    parent[0][v] = p;
    depth[v] = d;
    for (auto& e : g[v]) {
      if (e.to != p) dfs(g, e.to, v, d + 1);
    }
  }
  int get(int u, int v) {
    if (depth[u] > depth[v]) std::swap(u, v);
    for (int k = 0; k < log2_n; k++) {
      if ((depth[v] - depth[u]) >> k & 1) {
        v = parent[k][v];
      }
    }
    if (u == v) return u;
    for (int k = log2_n - 1; k >= 0; k--) {
      if (parent[k][u] != parent[k][v]) {
        u = parent[k][u];
        v = parent[k][v];
      }
    }
    return parent[0][u];
  }
};
void visit(const vector<vector<Edge>>& g, int v, vector<vector<long long>>& scc,
           stack<long long>& S, vector<long long>& inS, vector<long long>& low,
           vector<long long>& num, int& time) {
  low[v] = num[v] = ++time;
  S.push(v);
  inS[v] = true;
  for (decltype((g[v]).begin()) e = (g[v]).begin(); e != (g[v]).end(); ++e) {
    int w = e->to;
    if (num[w] == 0) {
      visit(g, w, scc, S, inS, low, num, time);
      low[v] = min(low[v], low[w]);
    } else if (inS[w])
      low[v] = min(low[v], num[w]);
  }
  if (low[v] == num[v]) {
    scc.push_back(vector<long long>());
    while (1) {
      int w = S.top();
      S.pop();
      inS[w] = false;
      scc.back().push_back(w);
      if (v == w) break;
    }
  }
}
void stronglyConnectedComponents(const vector<vector<Edge>>& g,
                                 vector<vector<long long>>& scc) {
  const int n = g.size();
  vector<long long> num(n), low(n);
  stack<long long> S;
  vector<long long> inS(n);
  int time = 0;
  for (int(u) = (int)(0); u < (int)(n); u++)
    if (num[u] == 0) visit(g, u, scc, S, inS, low, num, time);
}
class UnionFind {
  vector<int> data;
  long long num;

 public:
  UnionFind(int size) : data(size, -1), num(size) {}
  bool unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x]) swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    num -= (x != y);
    return x != y;
  }
  bool findSet(int x, int y) { return root(x) == root(y); }
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  int size(int x) { return -data[root(x)]; }
  int numSet() { return num; }
};
class SumSegTree {
 private:
  long long _sum(long long a, long long b, long long k, long long l,
                 long long r) {
    if (r <= a || b <= l) return 0;
    if (a <= l && r <= b)
      return dat[k];
    else {
      long long s1 = _sum(a, b, 2 * k + 1, l, (l + r) / 2);
      long long s2 = _sum(a, b, 2 * k + 2, (l + r) / 2, r);
      return s1 + s2;
    }
  }

 public:
  long long n, height;
  vector<long long> dat;
  SumSegTree(long long _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<long long>(2 * n - 1, 0);
  }
  void add(long long i, long long x) {
    i += n - 1;
    dat[i] += x;
    while (i > 0) {
      i = (i - 1) / 2;
      dat[i] += x;
    }
  }
  long long sum(long long a, long long b) { return _sum(a, b, 0, 0, n); }
};
class RmqTree {
 private:
  long long _find(long long a, long long b, long long k, long long l,
                  long long r) {
    if (r <= a || b <= l) return INF;
    if (a <= l && r <= b)
      return dat[k];
    else {
      long long s1 = _find(a, b, 2 * k + 1, l, (l + r) / 2);
      long long s2 = _find(a, b, 2 * k + 2, (l + r) / 2, r);
      return min(s1, s2);
    }
  }

 public:
  long long n, height;
  vector<long long> dat;
  RmqTree(long long _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<long long>(2 * n - 1, INF);
  }
  void update(long long i, long long x) {
    i += n - 1;
    dat[i] = x;
    while (i > 0) {
      i = (i - 1) / 2;
      dat[i] = min(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }
  long long find(long long a, long long b) { return _find(a, b, 0, 0, n); }
};
void divisor(long long n, vector<long long>& ret) {
  for (long long i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n) ret.push_back(n / i);
    }
  }
  sort(ret.begin(), ret.end());
}
void divisor_prime(long long n, vector<pair<long long, long long>>& ret) {
  for (long long i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back({i, 0});
      while (n % i == 0) {
        n /= i;
        ret[ret.size() - 1].second++;
      }
    }
  }
  if (n != 1) ret.push_back({n, 1});
}
long long mod_pow(long long x, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1) res = res * x % mod;
    x = x * x % mod;
    n >>= 1;
  }
  return res;
}
long long mod_inv(long long x, long long mod) {
  return mod_pow(x, mod - 2, mod);
}
class Combination {
 public:
  vector<long long> fact;
  vector<long long> inv;
  long long mod;
  long long mod_inv(long long x) {
    long long n = mod - 2LL;
    long long res = 1LL;
    while (n > 0) {
      if (n & 1) res = res * x % mod;
      x = x * x % mod;
      n >>= 1;
    }
    return res;
  }
  long long nCr(long long n, long long r) {
    return ((fact[n] * inv[r] % mod) * inv[n - r]) % mod;
  }
  long long nPr(long long n, long long r) {
    return (fact[n] * inv[n - r]) % mod;
  }
  long long nHr(long long n, long long r) { return nCr(r + n - 1, r); }
  Combination(long long n, long long _mod) {
    mod = _mod;
    fact.resize(n + 1);
    fact[0] = 1;
    for (int(i) = (int)(0); i < (int)(n); i++) {
      fact[i + 1] = (fact[i] * (i + 1LL)) % mod;
    }
    inv.resize(n + 1);
    inv[n] = mod_inv(fact[n]);
    for (int i = n; i > 0; i--) {
      inv[i - 1] = inv[i] * i % mod;
    }
  }
};
long long gcd(long long m, long long n) {
  if (n == 0) return m;
  return gcd(n, m % n);
}
long long lcm(long long m, long long n) { return m / gcd(m, n) * n; }
long long popcount(long long x) {
  x = (x & 0x5555555555555555) + (x >> 1 & 0x5555555555555555);
  x = (x & 0x3333333333333333) + (x >> 2 & 0x3333333333333333);
  x = (x & 0x0F0F0F0F0F0F0F0F) + (x >> 4 & 0x0F0F0F0F0F0F0F0F);
  x = (x & 0x00FF00FF00FF00FF) + (x >> 8 & 0x00FF00FF00FF00FF);
  x = (x & 0x0000FFFF0000FFFF) + (x >> 16 & 0x0000FFFF0000FFFF);
  x = (x & 0x00000000FFFFFFFF) + (x >> 32 & 0x00000000FFFFFFFF);
  return x;
}
unsigned long long dp[110][110];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  vector<vector<long long>> last(n + 1, vector<long long>(26, -1));
  for (int(i) = (int)(0); i < (int)(n); i++) {
    long long ch = s[i] - 'a';
    last[i + 1][ch] = i;
    for (int(j) = (int)(0); j < (int)(26); j++)
      chmax(last[i + 1][j], last[i][j]);
  }
  dp[0][0] = 1;
  for (int(i) = (int)(0); i < (int)(n); i++) {
    for (int(j) = (int)(0); j < (int)(n); j++) {
      if (dp[i][j] == 0) continue;
      long long ch = s[i] - 'a';
      dp[i + 1][j] += dp[i][j];
      dp[i + 1][j + 1] +=
          dp[i][j] - (last[i][ch] == -1 ? 0 : dp[last[i][ch]][j]);
    }
  }
  unsigned long long ans = 0;
  unsigned long long sum = 0;
  for (int i = n; i >= 0; i--) {
    if (dp[n][i] + sum <= k)
      sum += dp[n][i], ans += (n - i) * dp[n][i];
    else
      ans += (n - i) * (k - sum), sum = k;
  }
  if (sum == k)
    cout << ans << endl;
  else
    cout << -1 << endl;
  return 0;
}

### Prompt

Construct a Cpp code solution to the problem outlined:

The only difference between the easy and the hard versions is constraints.

A subsequence is a string that can be derived from another string by deleting some or no symbols without changing the order of the remaining symbols. Characters to be deleted are not required to go successively, there can be any gaps between them. For example, for the string "abaca" the following strings are subsequences: "abaca", "aba", "aaa", "a" and "" (empty string). But the following strings are not subsequences: "aabaca", "cb" and "bcaa".

You are given a string s consisting of n lowercase Latin letters.

In one move you can take any subsequence t of the given string and add it to the set S. The set S can't contain duplicates. This move costs n - |t|, where |t| is the length of the added subsequence (i.e. the price equals to the number of the deleted characters).

Your task is to find out the minimum possible total cost to obtain a set S of size k or report that it is impossible to do so.

Input

The first line of the input contains two integers n and k (1 ≤ n ≤ 100, 1 ≤ k ≤ 10^{12}) — the length of the string and the size of the set, correspondingly.

The second line of the input contains a string s consisting of n lowercase Latin letters.

Output

Print one integer — if it is impossible to obtain the set S of size k, print -1. Otherwise, print the minimum possible total cost to do it.

Examples

Input


4 5
asdf


Output


4


Input


5 6
aaaaa


Output


15


Input


5 7
aaaaa


Output


-1


Input


10 100
ajihiushda


Output


233

Note

In the first example we can generate S = { "asdf", "asd", "adf", "asf", "sdf" }. The cost of the first element in S is 0 and the cost of the others is 1. So the total cost of S is 4.

### Response

```cpp
#include <bits/stdc++.h>
using namespace std;
const long long INF = 1LL << 60;
const long double EPS = 1e-10;
string operator*(const string& s, int k) {
  if (k == 0) return "";
  string p = (s + s) * (k / 2);
  if (k % 2 == 1) p += s;
  return p;
}
template <class T>
inline bool chmin(T& a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T& a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
struct Edge {
  long long to, cap, rev;
  Edge(long long _to, long long _cap, long long _rev) {
    to = _to;
    cap = _cap;
    rev = _rev;
  }
};
void add_edge(vector<vector<Edge>>& G, long long from, long long to,
              long long cap, bool revFlag, long long revCap) {
  G[from].push_back(Edge(to, cap, (long long)G[to].size()));
  if (revFlag)
    G[to].push_back(Edge(from, revCap, (long long)G[from].size() - 1));
}
long long max_flow_dfs(vector<vector<Edge>>& G, long long v, long long t,
                       long long f, vector<bool>& used) {
  if (v == t) return f;
  used[v] = true;
  for (int i = 0; i < G[v].size(); ++i) {
    Edge& e = G[v][i];
    if (!used[e.to] && e.cap > 0) {
      long long d = max_flow_dfs(G, e.to, t, min(f, e.cap), used);
      if (d > 0) {
        e.cap -= d;
        G[e.to][e.rev].cap += d;
        return d;
      }
    }
  }
  return 0;
}
long long max_flow(vector<vector<Edge>>& G, long long s, long long t) {
  long long flow = 0;
  for (;;) {
    vector<bool> used(G.size());
    for (int(i) = (int)(0); i < (int)(used.size()); i++) used[i] = false;
    long long f = max_flow_dfs(G, s, t, INF, used);
    if (f == 0) {
      return flow;
    }
    flow += f;
  }
}
void BellmanFord(vector<vector<Edge>>& G, long long s, vector<long long>& d,
                 vector<long long>& negative) {
  d.resize(G.size());
  negative.resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) d[i] = INF;
  for (int(i) = (int)(0); i < (int)(d.size()); i++) negative[i] = false;
  d[s] = 0;
  for (int(k) = (int)(0); k < (int)(G.size() - 1); k++) {
    for (int(i) = (int)(0); i < (int)(G.size()); i++) {
      for (int(j) = (int)(0); j < (int)(G[i].size()); j++) {
        if (d[i] != INF && d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
        }
      }
    }
  }
  for (int(k) = (int)(0); k < (int)(G.size() - 1); k++) {
    for (int(i) = (int)(0); i < (int)(G.size()); i++) {
      for (int(j) = (int)(0); j < (int)(G[i].size()); j++) {
        if (d[i] != INF && d[G[i][j].to] > d[i] + G[i][j].cap) {
          d[G[i][j].to] = d[i] + G[i][j].cap;
          negative[G[i][j].to] = true;
        }
        if (negative[i] == true) negative[G[i][j].to] = true;
      }
    }
  }
}
void Dijkstra(vector<vector<Edge>>& G, long long s, vector<long long>& d) {
  d.resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) d[i] = INF;
  d[s] = 0;
  priority_queue<pair<long long, long long>, vector<pair<long long, long long>>,
                 greater<pair<long long, long long>>>
      q;
  q.push(make_pair(0, s));
  while (!q.empty()) {
    pair<long long, long long> a = q.top();
    q.pop();
    if (d[a.second] < a.first) continue;
    for (int(i) = (int)(0); i < (int)(G[a.second].size()); i++) {
      Edge e = G[a.second][i];
      if (d[e.to] > d[a.second] + e.cap) {
        d[e.to] = d[a.second] + e.cap;
        q.push(make_pair(d[e.to], e.to));
      }
    }
  }
}
void WarshallFloyd(vector<vector<Edge>>& G, vector<vector<long long>>& d) {
  d.resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) d[i].resize(G.size());
  for (int(i) = (int)(0); i < (int)(d.size()); i++) {
    for (int(j) = (int)(0); j < (int)(d[i].size()); j++) {
      d[i][j] = ((i != j) ? INF : 0);
    }
  }
  for (int(i) = (int)(0); i < (int)(G.size()); i++) {
    for (int(j) = (int)(0); j < (int)(G[i].size()); j++) {
      chmin(d[i][G[i][j].to], G[i][j].cap);
    }
  }
  for (int(i) = (int)(0); i < (int)(G.size()); i++) {
    for (int(j) = (int)(0); j < (int)(G.size()); j++) {
      for (int(k) = (int)(0); k < (int)(G.size()); k++) {
        chmin(d[j][k], d[j][i] + d[i][k]);
      }
    }
  }
}
bool tsort(vector<vector<Edge>>& graph, vector<int>& order) {
  int n = graph.size(), k = 0;
  vector<long long> in(n);
  for (auto& es : graph)
    for (auto& e : es) in[e.to]++;
  priority_queue<long long, vector<long long>, greater<long long>> que;
  for (int(i) = (int)(0); i < (int)(n); i++)
    if (in[i] == 0) que.push(i);
  while (que.size()) {
    int v = que.top();
    que.pop();
    order.push_back(v);
    for (auto& e : graph[v])
      if (--in[e.to] == 0) que.push(e.to);
  }
  if (order.size() != n)
    return false;
  else
    return true;
}
class lca {
 public:
  const int n = 0;
  const int log2_n = 0;
  std::vector<std::vector<int>> parent;
  std::vector<int> depth;
  lca() {}
  lca(const vector<vector<Edge>>& g, int root)
      : n(g.size()),
        log2_n(log2(n) + 1),
        parent(log2_n, std::vector<int>(n)),
        depth(n) {
    dfs(g, root, -1, 0);
    for (int k = 0; k + 1 < log2_n; k++) {
      for (int v = 0; v < (int)g.size(); v++) {
        if (parent[k][v] < 0)
          parent[k + 1][v] = -1;
        else
          parent[k + 1][v] = parent[k][parent[k][v]];
      }
    }
  }
  void dfs(const vector<vector<Edge>>& g, int v, int p, int d) {
    parent[0][v] = p;
    depth[v] = d;
    for (auto& e : g[v]) {
      if (e.to != p) dfs(g, e.to, v, d + 1);
    }
  }
  int get(int u, int v) {
    if (depth[u] > depth[v]) std::swap(u, v);
    for (int k = 0; k < log2_n; k++) {
      if ((depth[v] - depth[u]) >> k & 1) {
        v = parent[k][v];
      }
    }
    if (u == v) return u;
    for (int k = log2_n - 1; k >= 0; k--) {
      if (parent[k][u] != parent[k][v]) {
        u = parent[k][u];
        v = parent[k][v];
      }
    }
    return parent[0][u];
  }
};
void visit(const vector<vector<Edge>>& g, int v, vector<vector<long long>>& scc,
           stack<long long>& S, vector<long long>& inS, vector<long long>& low,
           vector<long long>& num, int& time) {
  low[v] = num[v] = ++time;
  S.push(v);
  inS[v] = true;
  for (decltype((g[v]).begin()) e = (g[v]).begin(); e != (g[v]).end(); ++e) {
    int w = e->to;
    if (num[w] == 0) {
      visit(g, w, scc, S, inS, low, num, time);
      low[v] = min(low[v], low[w]);
    } else if (inS[w])
      low[v] = min(low[v], num[w]);
  }
  if (low[v] == num[v]) {
    scc.push_back(vector<long long>());
    while (1) {
      int w = S.top();
      S.pop();
      inS[w] = false;
      scc.back().push_back(w);
      if (v == w) break;
    }
  }
}
void stronglyConnectedComponents(const vector<vector<Edge>>& g,
                                 vector<vector<long long>>& scc) {
  const int n = g.size();
  vector<long long> num(n), low(n);
  stack<long long> S;
  vector<long long> inS(n);
  int time = 0;
  for (int(u) = (int)(0); u < (int)(n); u++)
    if (num[u] == 0) visit(g, u, scc, S, inS, low, num, time);
}
class UnionFind {
  vector<int> data;
  long long num;

 public:
  UnionFind(int size) : data(size, -1), num(size) {}
  bool unite(int x, int y) {
    x = root(x);
    y = root(y);
    if (x != y) {
      if (data[y] < data[x]) swap(x, y);
      data[x] += data[y];
      data[y] = x;
    }
    num -= (x != y);
    return x != y;
  }
  bool findSet(int x, int y) { return root(x) == root(y); }
  int root(int x) { return data[x] < 0 ? x : data[x] = root(data[x]); }
  int size(int x) { return -data[root(x)]; }
  int numSet() { return num; }
};
class SumSegTree {
 private:
  long long _sum(long long a, long long b, long long k, long long l,
                 long long r) {
    if (r <= a || b <= l) return 0;
    if (a <= l && r <= b)
      return dat[k];
    else {
      long long s1 = _sum(a, b, 2 * k + 1, l, (l + r) / 2);
      long long s2 = _sum(a, b, 2 * k + 2, (l + r) / 2, r);
      return s1 + s2;
    }
  }

 public:
  long long n, height;
  vector<long long> dat;
  SumSegTree(long long _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<long long>(2 * n - 1, 0);
  }
  void add(long long i, long long x) {
    i += n - 1;
    dat[i] += x;
    while (i > 0) {
      i = (i - 1) / 2;
      dat[i] += x;
    }
  }
  long long sum(long long a, long long b) { return _sum(a, b, 0, 0, n); }
};
class RmqTree {
 private:
  long long _find(long long a, long long b, long long k, long long l,
                  long long r) {
    if (r <= a || b <= l) return INF;
    if (a <= l && r <= b)
      return dat[k];
    else {
      long long s1 = _find(a, b, 2 * k + 1, l, (l + r) / 2);
      long long s2 = _find(a, b, 2 * k + 2, (l + r) / 2, r);
      return min(s1, s2);
    }
  }

 public:
  long long n, height;
  vector<long long> dat;
  RmqTree(long long _n) {
    n = 1;
    height = 1;
    while (n < _n) {
      n *= 2;
      height++;
    }
    dat = vector<long long>(2 * n - 1, INF);
  }
  void update(long long i, long long x) {
    i += n - 1;
    dat[i] = x;
    while (i > 0) {
      i = (i - 1) / 2;
      dat[i] = min(dat[i * 2 + 1], dat[i * 2 + 2]);
    }
  }
  long long find(long long a, long long b) { return _find(a, b, 0, 0, n); }
};
void divisor(long long n, vector<long long>& ret) {
  for (long long i = 1; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back(i);
      if (i * i != n) ret.push_back(n / i);
    }
  }
  sort(ret.begin(), ret.end());
}
void divisor_prime(long long n, vector<pair<long long, long long>>& ret) {
  for (long long i = 2; i * i <= n; i++) {
    if (n % i == 0) {
      ret.push_back({i, 0});
      while (n % i == 0) {
        n /= i;
        ret[ret.size() - 1].second++;
      }
    }
  }
  if (n != 1) ret.push_back({n, 1});
}
long long mod_pow(long long x, long long n, long long mod) {
  long long res = 1;
  while (n > 0) {
    if (n & 1) res = res * x % mod;
    x = x * x % mod;
    n >>= 1;
  }
  return res;
}
long long mod_inv(long long x, long long mod) {
  return mod_pow(x, mod - 2, mod);
}
class Combination {
 public:
  vector<long long> fact;
  vector<long long> inv;
  long long mod;
  long long mod_inv(long long x) {
    long long n = mod - 2LL;
    long long res = 1LL;
    while (n > 0) {
      if (n & 1) res = res * x % mod;
      x = x * x % mod;
      n >>= 1;
    }
    return res;
  }
  long long nCr(long long n, long long r) {
    return ((fact[n] * inv[r] % mod) * inv[n - r]) % mod;
  }
  long long nPr(long long n, long long r) {
    return (fact[n] * inv[n - r]) % mod;
  }
  long long nHr(long long n, long long r) { return nCr(r + n - 1, r); }
  Combination(long long n, long long _mod) {
    mod = _mod;
    fact.resize(n + 1);
    fact[0] = 1;
    for (int(i) = (int)(0); i < (int)(n); i++) {
      fact[i + 1] = (fact[i] * (i + 1LL)) % mod;
    }
    inv.resize(n + 1);
    inv[n] = mod_inv(fact[n]);
    for (int i = n; i > 0; i--) {
      inv[i - 1] = inv[i] * i % mod;
    }
  }
};
long long gcd(long long m, long long n) {
  if (n == 0) return m;
  return gcd(n, m % n);
}
long long lcm(long long m, long long n) { return m / gcd(m, n) * n; }
long long popcount(long long x) {
  x = (x & 0x5555555555555555) + (x >> 1 & 0x5555555555555555);
  x = (x & 0x3333333333333333) + (x >> 2 & 0x3333333333333333);
  x = (x & 0x0F0F0F0F0F0F0F0F) + (x >> 4 & 0x0F0F0F0F0F0F0F0F);
  x = (x & 0x00FF00FF00FF00FF) + (x >> 8 & 0x00FF00FF00FF00FF);
  x = (x & 0x0000FFFF0000FFFF) + (x >> 16 & 0x0000FFFF0000FFFF);
  x = (x & 0x00000000FFFFFFFF) + (x >> 32 & 0x00000000FFFFFFFF);
  return x;
}
unsigned long long dp[110][110];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  vector<vector<long long>> last(n + 1, vector<long long>(26, -1));
  for (int(i) = (int)(0); i < (int)(n); i++) {
    long long ch = s[i] - 'a';
    last[i + 1][ch] = i;
    for (int(j) = (int)(0); j < (int)(26); j++)
      chmax(last[i + 1][j], last[i][j]);
  }
  dp[0][0] = 1;
  for (int(i) = (int)(0); i < (int)(n); i++) {
    for (int(j) = (int)(0); j < (int)(n); j++) {
      if (dp[i][j] == 0) continue;
      long long ch = s[i] - 'a';
      dp[i + 1][j] += dp[i][j];
      dp[i + 1][j + 1] +=
          dp[i][j] - (last[i][ch] == -1 ? 0 : dp[last[i][ch]][j]);
    }
  }
  unsigned long long ans = 0;
  unsigned long long sum = 0;
  for (int i = n; i >= 0; i--) {
    if (dp[n][i] + sum <= k)
      sum += dp[n][i], ans += (n - i) * dp[n][i];
    else
      ans += (n - i) * (k - sum), sum = k;
  }
  if (sum == k)
    cout << ans << endl;
  else
    cout << -1 << endl;
  return 0;
}

```