You are given a graph with n nodes and m directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is 3. Your task is find a path whose value is the largest.

Input

The first line contains two positive integers n, m (1 ≤ n, m ≤ 300 000), denoting that the graph has n nodes and m directed edges.

The second line contains a string s with only lowercase English letters. The i-th character is the letter assigned to the i-th node.

Then m lines follow. Each line contains two integers x, y (1 ≤ x, y ≤ n), describing a directed edge from x to y. Note that x can be equal to y and there can be multiple edges between x and y. Also the graph can be not connected.

Output

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

Examples

Input

5 4
abaca
1 2
1 3
3 4
4 5


Output

3


Input

6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4


Output

-1


Input

10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7


Output

4

Note

In the first sample, the path with largest value is 1 → 3 → 4 → 5. The value is 3 because the letter 'a' appears 3 times.
#include <bits/stdc++.h>
using namespace std;
int q, n;
long long int ans, sum;
long long int gcd(long long int a, long long int b) {
  if (b == 0) {
    return a;
  }
  return gcd(b, a % b);
}
long long int powe(long long int a, long long int b) {
  long long int res = 1;
  while (b > 0) {
    if (b % 2 == 1) {
      res = (res * a);
    }
    a = (a * a);
    b /= 2;
  }
  return res;
}
long long int power(long long int a, long long int b, long long int M) {
  a %= M;
  long long int res = 1;
  while (b > 0) {
    if (b % 2 == 1) {
      res = (res * a) % M;
    }
    a = (a * a) % M;
    b /= 2;
  }
  return res;
}
long long int extendedEuclid(long long int A, long long int B, long long int &x,
                             long long int &y) {
  if (B == 0) {
    x = 1;
    y = 0;
    return A;
  } else {
    long long int x1, y1;
    long long int gcd = extendedEuclid(B, A % B, x1, y1);
    y = x1 - (A / B) * y1;
    x = y1;
    return gcd;
  }
}
long long int mi(long long int A, long long int M) {
  long long int x, y;
  extendedEuclid(A, M, x, y);
  if (x < 0) {
    x += (long long int)1000000007;
  }
  return x;
}
vector<bool> sieve(1000000, true);
void Sieve() {
  sieve[0] = false;
  sieve[1] = false;
  for (long long int i = 2; i * i <= 1000000; i++) {
    if (sieve[i] == true) {
      for (long long int j = i * i; j < 1000000; j += i) sieve[j] = false;
    }
  }
}
vector<long long int> sieve_spf;
void Sieve_spf() {
  const long long int n = 1e6 + 5;
  sieve_spf.resize(n);
  for (int i = 0; i < n; i++) sieve_spf[i] = i;
  sieve_spf[0] = -1;
  sieve_spf[1] = 1;
  for (int i = 2; i <= n; i += 2) sieve_spf[i] = 2;
  for (int i = 3; i <= n; i += 2)
    if (sieve_spf[i] == i)
      for (int j = i * i; j <= n; j += i)
        if (sieve_spf[j] == j) sieve_spf[j] = i;
}
bool isUpcase(char c) {
  if (c >= 'A' && c <= 'Z')
    return true;
  else
    return false;
}
bool islowcase(char c) {
  if (c >= 'a' && c <= 'z')
    return true;
  else
    return false;
}
void the_happiest_place_on_earth() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
}
bool comp(const pair<int, int> &a, const pair<int, int> &b) {
  if (a.first > b.first)
    return true;
  else if (a.first == b.first && a.second < b.second)
    return true;
  else
    return false;
}
const int N = 310055;
long long int b[N], a[N], dis[N], p[N];
vector<int> graph[N], graph1[N];
bool vis[N];
int dag;
void gra(long long int m) {
  for (int i = 0; i < m; i++) {
    long long int x, y;
    cin >> x >> y;
    x--, y--;
    graph[x].push_back(y);
  }
}
string second;
long long int color[N], dp[N][30];
void dfs(int node) {
  vis[node] = true;
  color[node] = 1;
  for (auto x : graph[node]) {
    if (color[x] == 1) {
      cout << -1;
      exit(0);
    }
    if (!vis[x]) dfs(x);
    for (int i = 0; i < 26; i++) dp[node][i] = max(dp[node][i], dp[x][i]);
  }
  dp[node][second[node] - 'a']++;
  for (int i = 0; i < 26; i++) ans = max(dp[node][i], ans);
  color[node] = 0;
}
void testcase() {
  long long int m;
  cin >> n >> m >> second;
  gra(m);
  memset(color, 0, sizeof(color));
  for (int i = 0; i < n; i++)
    if (!vis[i]) dfs(i);
  cout << ans;
  return;
}
int main() {
  the_happiest_place_on_earth();
  q = 1;
  while (q--) {
    testcase();
  }
  cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC
       << "ms\n";
  return 0;
}
