#include <bits/stdc++.h>
using namespace std;
char ch[300005];
int n, m, dp[300005][26];
bool in_stack[300005], vis[300005];
vector<vector<int> > adj;
bool check(int u) {
  if (in_stack[u])
    return true;
  else if (vis[u])
    return false;
  in_stack[u] = vis[u] = true;
  for (int v : adj[u]) {
    if (check(v)) return true;
  }
  in_stack[u] = false;
  return false;
}
int solve(int u, int c) {
  if (dp[u][c] == -1) {
    int ans = ((ch[u] - 'a') == c);
    int cv = ans;
    for (int v : adj[u]) ans = max(ans, cv + solve(v, c));
    dp[u][c] = ans;
  }
  return dp[u][c];
}
bool cycle() {
  for (int i = 0; i < n; i++)
    if (check(i)) return true;
  return false;
}
int main() {
  scanf("%d", &n), scanf("%d", &m);
  scanf("%s", ch);
  adj = vector<vector<int> >(n, vector<int>());
  for (int i = 0; i < m; i++) {
    int u, v;
    scanf("%d", &u), scanf("%d", &v);
    adj[u - 1].push_back(v - 1);
  }
  memset(dp, -1, sizeof(dp));
  memset(in_stack, 0, sizeof(in_stack));
  memset(vis, 0, sizeof(vis));
  if (cycle())
    printf("-1\n");
  else {
    int ans = 0;
    for (int i = 0; i < n; i++)
      for (int j = 0; j < 26; j++) ans = max(ans, solve(i, j));
    printf("%d\n", ans);
  }
  return 0;
}
