#include <bits/stdc++.h>
using namespace std;
const int N = (int)3e5;
int n, timer, top;
vector<int> G[N + 5];
int dfsn[N + 5], pila[N + 5], inpila[N + 5], comp[N + 5];
int dfs(int u) {
  int low = dfsn[u] = ++timer;
  inpila[pila[top++] = u] = 1;
  for (int v : G[u]) {
    if (dfsn[v] == 0)
      low = min(low, dfs(v));
    else if (inpila[v])
      low = min(low, dfsn[v]);
  }
  if (low == dfsn[u]) {
    int fin;
    do {
      fin = pila[--top];
      inpila[fin] = 0;
      comp[fin] = u;
    } while (fin != u);
  }
  return low;
}
void SCC() {
  memset(dfsn, 0, sizeof(dfsn));
  top = timer = 0;
  for (int i = 0; i < n; i++)
    if (!dfsn[i]) dfs(i);
}
void add(int u, int v) { G[u].push_back(v); }
bool used[N][26];
int memo[N][26];
string letters;
int dp(int u, int target) {
  if (used[u][target]) {
    return memo[u][target];
  }
  used[u][target] = true;
  int &ans = memo[u][target] = (letters[u] - 'a' == target);
  for (int i = 0; i < ((int)(G[u]).size()); i++) {
    int v = G[u][i];
    ans = max(ans, (letters[u] - 'a' == target) + dp(v, target));
  }
  return ans;
}
int main() {
  ios_base ::sync_with_stdio(false);
  int m;
  while (cin >> n >> m) {
    cin >> letters;
    for (int i = 0; i < n; i++) {
      G[i].clear();
    }
    vector<int> indeg(n);
    bool hasCycle = false;
    for (int i = 0; i < m; i++) {
      int u, v;
      cin >> u >> v;
      u--, v--;
      if (u == v) {
        hasCycle = true;
      }
      add(u, v);
      indeg[v]++;
    }
    SCC();
    for (int i = 0; i < n; i++) {
      if (comp[i] != i) {
        hasCycle = true;
      }
    }
    if (hasCycle) {
      cout << -1 << '\n';
      continue;
    }
    memset(used, false, sizeof(used));
    int ans = 0;
    for (int letter = 0; letter < 26; letter++) {
      for (int u = 0; u < n; u++) {
        if (indeg[u] == 0) {
          ans = max(ans, dp(u, letter));
        }
      }
    }
    cout << ans << '\n';
  }
}
