#include <bits/stdc++.h>
using namespace std;
const int N = (int)3e5 + 11;
int n, m, dp[N][30], cnt[N];
vector<int> g[N], q[N];
int u[N];
bool cyc;
string s = "#";
void dfs(int v) {
  u[v] = 1;
  for (int i = 0; i < g[v].size(); i++) {
    int to = g[v][i];
    if (u[to] == 1 or cyc) {
      cyc = 1;
      return;
    }
    if (u[to] == 0) dfs(to);
  }
  u[v] = 2;
}
int main() {
  ;
  ios_base::sync_with_stdio(0);
  cin.tie();
  cout.tie();
  ;
  cin >> n >> m;
  string q1;
  cin >> q1;
  s += q1;
  for (int i = 1; i <= m; i++) {
    int x, y;
    cin >> x >> y;
    g[x].push_back(y);
    q[y].push_back(x);
    if (x == y) cyc = 1;
    cnt[y]++;
  }
  bool op = 1;
  for (int i = 1; i <= n; i++) {
    if (u[i] == 0 and q[i].empty()) dfs(i), op = 0;
    if (cyc) {
      cout << "-1\n";
      return 0;
    }
  }
  if (op) {
    cout << "-1";
    return 0;
  }
  memset(u, 0, sizeof(u));
  queue<int> w;
  for (int i = 1; i <= n; i++)
    if (q[i].empty()) {
      u[i] = 1;
      w.push(i);
    }
  while (!w.empty()) {
    int v = w.front();
    bool ok = 1;
    for (int i = 0; i < q[v].size(); i++)
      for (int j = 0; j < 26; j++) {
        int to = q[v][i];
        dp[v][j] = max(dp[v][j], dp[to][j]);
      }
    dp[v][s[v] - 'a']++;
    for (int i = 0; i < g[v].size(); i++) {
      int to = g[v][i];
      if (!u[to]) {
        cnt[to]--;
        if (cnt[to]) continue;
        u[to] = 1;
        w.push(to);
      }
    }
    w.pop();
  }
  int ans = 0;
  for (int i = 1; i <= n; i++)
    for (int j = 0; j < 26; j++) ans = max(ans, dp[i][j]);
  cout << ans;
  return 0;
}
