You are given a graph with n nodes and m directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is 3. Your task is find a path whose value is the largest.

Input

The first line contains two positive integers n, m (1 ≤ n, m ≤ 300 000), denoting that the graph has n nodes and m directed edges.

The second line contains a string s with only lowercase English letters. The i-th character is the letter assigned to the i-th node.

Then m lines follow. Each line contains two integers x, y (1 ≤ x, y ≤ n), describing a directed edge from x to y. Note that x can be equal to y and there can be multiple edges between x and y. Also the graph can be not connected.

Output

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

Examples

Input

5 4
abaca
1 2
1 3
3 4
4 5


Output

3


Input

6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4


Output

-1


Input

10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7


Output

4

Note

In the first sample, the path with largest value is 1 → 3 → 4 → 5. The value is 3 because the letter 'a' appears 3 times.
#include <bits/stdc++.h>
using namespace std;
const int maxn = 3e5 + 100;
int first[maxn], des[maxn], nxt[maxn], tot;
int a[maxn];
int dp[maxn][26];
int d[maxn];
int m, n;
int vis[maxn];
int findCir(int node) {
  if (vis[node] == 2) return 0;
  if (vis[node] == 1) return 1;
  vis[node] = 1;
  for (int t = first[node]; t; t = nxt[t]) {
    int v = des[t];
    if (findCir(v)) return 1;
  }
  vis[node] = 2;
  return 0;
}
inline void addEdge(int u, int v) {
  tot++;
  des[tot] = v;
  nxt[tot] = first[u];
  first[u] = tot;
  d[v]++;
}
char s[maxn];
void input() {
  scanf("%d%d", &n, &m);
  scanf("%s", s + 1);
  for (int i = 1; i <= n; i++) {
    a[i] = s[i] - 'a';
  }
  for (int i = 1; i <= m; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    addEdge(v, u);
  }
}
queue<int> Q;
void solve() {
  for (int i = 1; i <= n; i++) {
    if (!vis[i]) {
      if (findCir(i)) {
        cout << -1 << endl;
        return;
      }
    }
  }
  for (int i = 1; i <= n; i++) {
    if (!d[i]) {
      Q.push(i);
      int id = a[i];
    }
  }
  while (!Q.empty()) {
    int u = Q.front();
    Q.pop();
    dp[u][a[u]]++;
    for (int t = first[u]; t; t = nxt[t]) {
      int v = des[t];
      for (int i = 0; i < 26; i++) {
        dp[v][i] = max(dp[v][i], dp[u][i]);
      }
      d[v]--;
      if (d[v] == 0) Q.push(v);
    }
  }
  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 << endl;
}
int main() {
  input();
  solve();
  return 0;
}
