#include <bits/stdc++.h>
using namespace std;
vector<int> g[300005];
int cnt[300005];
int vis[300005];
int ans[300001][26];
int res = 0;
int main() {
  queue<int> q;
  int n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  int x, y;
  while (m--) {
    scanf("%d%d", &x, &y);
    x--, y--;
    g[x].push_back(y);
    cnt[y]++;
  }
  for (int i = 0; i < s.size(); i++) {
    ans[i][s[i] - 'a']++;
    if (!cnt[i]) q.push(i);
  }
  while (!q.empty()) {
    int i = q.front();
    q.pop();
    for (auto v : g[i]) {
      cnt[v]--;
      for (int j = 0; j < 26; j++) {
        ans[v][j] = max(ans[v][j], ans[i][j] + (s[v] == j + 'a'));
        res = max(res, ans[v][j]);
      }
      if (!cnt[v]) q.push(v);
    }
  }
  if (*max_element(cnt, cnt + s.size())) return cout << -1, 0;
  cout << res;
  return 0;
}
