#include <bits/stdc++.h>
const int mod = 1e9 + 7;
const int maxn = 3e5 + 5;
const long long inf = 0x3f3f3f3f;
using namespace std;
int n, m;
int ru[maxn];
int ans;
int cnt[26];
int tmp[26];
int dp[maxn][26];
vector<int> g[maxn];
vector<int> qi;
char a[maxn];
bool vis[maxn];
bool judge() {
  queue<int> q;
  int sum = 0;
  for (int i = 1; i <= n; i++) {
    if (ru[i] == 0) q.push(i), sum++;
  }
  while (!q.empty()) {
    int s = q.front();
    q.pop();
    for (int i = 0; i < g[s].size(); i++) {
      ru[g[s][i]]--;
      if (ru[g[s][i]] == 0) sum++, q.push(g[s][i]);
    }
  }
  return sum == n;
}
void dfs(int v) {
  if (vis[v]) {
    for (int i = 0; i < 26; i++) {
      cnt[i] = max(cnt[i], dp[v][i] + tmp[i]);
    }
    return;
  }
  vis[v] = true;
  if (g[v].size() == 0) {
    tmp[a[v] - 'a']++;
    dp[v][a[v] - 'a']++;
    for (int i = 0; i < 26; i++) {
      cnt[i] = max(cnt[i], tmp[i]);
    }
    tmp[a[v] - 'a']--;
    return;
  }
  for (int i = 0; i < g[v].size(); i++) {
    tmp[a[v] - 'a']++;
    dfs(g[v][i]);
    for (int j = 0; j < 26; j++) {
      if (j == a[v] - 'a')
        dp[v][j] = max(dp[v][j], dp[g[v][i]][j] + 1);
      else
        dp[v][j] = max(dp[v][j], dp[g[v][i]][j]);
    }
    tmp[a[v] - 'a']--;
  }
}
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> m;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  while (m--) {
    int x, y;
    cin >> x >> y;
    ru[y]++;
    g[x].push_back(y);
  }
  for (int i = 1; i <= n; i++) {
    if (ru[i] == 0) qi.push_back(i);
  }
  if (!judge()) {
    cout << -1 << endl;
    return 0;
  }
  for (int j = 0; j < qi.size(); j++) {
    dfs(qi[j]);
  }
  for (int i = 0; i < qi.size(); i++) {
    for (int j = 0; j < 26; j++) {
      ans = max(ans, dp[qi[i]][j]);
    }
  }
  cout << ans << endl;
  return 0;
}
