#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const long long int inf = 1e17;
bool cmp(pair<long long int, long long int>& a,
         pair<long long int, long long int>& b) {
  if (a.first != b.first)
    return a.first > b.first;
  else
    return a.second < b.second;
}
vector<long long int> parent, size, visited, a, b;
vector<long long int> ans, xv;
vector<vector<long long int>> adj;
long long int modval(long long int n) { return n % mod; }
long long int modadd(long long int a, long long int b) {
  return modval(modval(a) + modval(mod + modval(b)));
}
long long int modmul(long long int a, long long int b) {
  return modval(modval(a) * modval(b));
}
long long int fast_expo(long long int base, long long int power,
                        long long int ans) {
  if (power > 0) {
    if (power & 1) ans = modmul(ans, base);
    power >>= 1, base = modmul(base, base);
    return fast_expo(base, power, ans);
  } else
    return ans;
}
long long int find_set(long long int v) {
  if (v == parent[v]) return v;
  return find_set(parent[v]);
}
void make_set(long long int v) {
  parent[v] = v;
  size[v] = 1;
}
void union_sets(long long int a, long long int b) {
  a = find_set(a);
  b = find_set(b);
  if (a != b) {
    if (size[a] < size[b]) swap(a, b);
    parent[b] = a;
    size[a] += size[b];
  }
}
void ceasars_legions() {
  long long int n, k;
  scanf("%lld %lld", &n, &k);
  if (n < k or (k == 1 and n != 1))
    printf("-1");
  else {
    for (long long int i = 0; i < n - k; i++) {
      if (i & 1)
        printf("b");
      else
        printf("a");
    }
    string s = "";
    for (long long int i = 0; i < k; i++) s += (char)('a' + i);
    if ((n - k) & 1) {
      char c = s[0];
      s[0] = s[1];
      s[1] = c;
    }
    for (auto c : s) cout << c;
  }
}
int32_t main() {
  long long int t = 1;
  while (t--) ceasars_legions();
  return 0;
}

### Prompt

Construct a Cpp code solution to the problem outlined:

Little penguin Polo adores strings. But most of all he adores strings of length n.

One day he wanted to find a string that meets the following conditions:

  1. The string consists of n lowercase English letters (that is, the string's length equals n), exactly k of these letters are distinct. 
  2. No two neighbouring letters of a string coincide; that is, if we represent a string as s = s1s2... sn, then the following inequality holds, si ≠ si + 1(1 ≤ i < n). 
  3. Among all strings that meet points 1 and 2, the required string is lexicographically smallest. 



Help him find such string or state that such string doesn't exist.

String x = x1x2... xp is lexicographically less than string y = y1y2... yq, if either p < q and x1 = y1, x2 = y2, ... , xp = yp, or there is such number r (r < p, r < q), that x1 = y1, x2 = y2, ... , xr = yr and xr + 1 < yr + 1. The characters of the strings are compared by their ASCII codes.

Input

A single line contains two positive integers n and k (1 ≤ n ≤ 106, 1 ≤ k ≤ 26) — the string's length and the number of distinct letters.

Output

In a single line print the required string. If there isn't such string, print "-1" (without the quotes).

Examples

Input

7 4


Output

ababacd


Input

4 7


Output

-1

### Response

```cpp
#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
const long long int inf = 1e17;
bool cmp(pair<long long int, long long int>& a,
         pair<long long int, long long int>& b) {
  if (a.first != b.first)
    return a.first > b.first;
  else
    return a.second < b.second;
}
vector<long long int> parent, size, visited, a, b;
vector<long long int> ans, xv;
vector<vector<long long int>> adj;
long long int modval(long long int n) { return n % mod; }
long long int modadd(long long int a, long long int b) {
  return modval(modval(a) + modval(mod + modval(b)));
}
long long int modmul(long long int a, long long int b) {
  return modval(modval(a) * modval(b));
}
long long int fast_expo(long long int base, long long int power,
                        long long int ans) {
  if (power > 0) {
    if (power & 1) ans = modmul(ans, base);
    power >>= 1, base = modmul(base, base);
    return fast_expo(base, power, ans);
  } else
    return ans;
}
long long int find_set(long long int v) {
  if (v == parent[v]) return v;
  return find_set(parent[v]);
}
void make_set(long long int v) {
  parent[v] = v;
  size[v] = 1;
}
void union_sets(long long int a, long long int b) {
  a = find_set(a);
  b = find_set(b);
  if (a != b) {
    if (size[a] < size[b]) swap(a, b);
    parent[b] = a;
    size[a] += size[b];
  }
}
void ceasars_legions() {
  long long int n, k;
  scanf("%lld %lld", &n, &k);
  if (n < k or (k == 1 and n != 1))
    printf("-1");
  else {
    for (long long int i = 0; i < n - k; i++) {
      if (i & 1)
        printf("b");
      else
        printf("a");
    }
    string s = "";
    for (long long int i = 0; i < k; i++) s += (char)('a' + i);
    if ((n - k) & 1) {
      char c = s[0];
      s[0] = s[1];
      s[1] = c;
    }
    for (auto c : s) cout << c;
  }
}
int32_t main() {
  long long int t = 1;
  while (t--) ceasars_legions();
  return 0;
}

```