#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,sse3,sse4,popcnt,abm,mmx")
const int N = (int)5e5 + 7;
const int inf = (int)1e9 + 7;
const int mod = (int)1e9 + 7;
const long long linf = (long long)1e18 + 7;
const int dx[] = {-1, 0, 1, 0, 1, -1, -1, 1};
const int dy[] = {0, 1, 0, -1, 1, -1, 1, -1};
using namespace std;
int n, m;
int a[N];
vector<int> g[N], gr[N];
int was[N];
int dp[N][26];
vector<int> cmp;
void dfs(int v) {
  was[v] = 1;
  for (auto to : g[v]) {
    if (!was[to]) {
      dfs(to);
    } else if (was[to] == 1) {
      cout << "-1", exit(0);
    }
  }
  was[v] = 2;
}
void dfs1(int v) {
  was[v] = 1;
  for (auto to : g[v]) {
    if (!was[to]) dfs1(to);
  }
  cmp.push_back(v);
}
int calc(int v, int x) {
  if (~dp[v][x]) return dp[v][x];
  int res = 0;
  for (auto to : g[v]) {
    res = max(res, calc(to, x));
  }
  return dp[v][x] = res + (a[v] == x);
}
void solve() {
  for (int i = 1; i <= n; i++) {
    if (!was[i]) dfs(i);
  }
  memset(was, 0, sizeof(was));
  for (int i = 1; i <= n; i++) {
    if (!was[i]) {
      dfs1(i);
    }
  }
  int ans = 0;
  reverse(cmp.begin(), cmp.end());
  memset(dp, -1, sizeof(dp));
  for (int i = 0; i < 26; i++) {
    for (auto it : cmp) {
      ans = max(ans, calc(it, i));
    }
  }
  cout << ans;
}
int main() {
  ios_base ::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> n >> m;
  for (int i = 1; i <= n; i++) {
    char x;
    cin >> x;
    a[i] = x - 'a';
  }
  for (int i = 1; i <= m; i++) {
    int v, u;
    cin >> v >> u;
    if (v == u) cout << -1, exit(0);
    g[v].push_back(u);
  }
  solve();
  exit(0);
}
