#include <bits/stdc++.h>
using namespace std;
const int maxn = 300 * 1000 + 100;
const long long mod = 1e9 + 7;
const long long inf = 2 * 1e18;
int n, m, k;
int ans = -1;
char c[maxn];
vector<int> adj[maxn];
int cnt[30][maxn];
int d_in[maxn];
bool mark[maxn], mark1[maxn];
void dfs(int v) {
  if (mark[v]) {
    cout << -1;
    exit(0);
  } else
    mark[v] = true;
  if (mark1[v]) {
    mark[v] = false;
    return;
  } else
    mark1[v] = true;
  for (auto x : adj[v]) {
    dfs(x);
    for (int j = 0; j <= 26; j++) {
      cnt[j][v] = max(cnt[j][v], cnt[j][x]);
      ans = max(ans, cnt[j][v]);
    }
  }
  cnt[c[v] - 'a'][v]++;
  ans = max(ans, cnt[c[v] - 'a'][v]);
  mark[v] = false;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  string s;
  cin >> s;
  for (int i = 1; i <= n; i++) c[i] = s[i - 1];
  for (int i = 1; i <= m; i++) {
    int u, v;
    cin >> u >> v;
    adj[u].push_back(v);
    d_in[v]++;
  }
  for (int i = 1; i <= n; i++) {
    dfs(i);
  }
  cout << ans;
  return 0;
}
