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 = 300005;
int n, m;
char s[MAXN];
vector<int> adj[MAXN];
bool flag;
int freq[MAXN][26];
bool used[MAXN];
bool hai[MAXN];
void topological_sort(int node) {
  used[node] = true;
  hai[node] = true;
  for (auto it : adj[node]) {
    if (!used[it]) {
      topological_sort(it);
    } else {
      if (hai[it]) flag = true;
    }
    for (int i = 0; i < 26; ++i)
      freq[node][i] = max(freq[node][i], freq[it][i]);
  }
  freq[node][s[node] - 'a']++;
  hai[node] = false;
}
int main() {
  int i, j, a, b;
  scanf("%d%d", &n, &m);
  scanf("%s", s);
  for (i = 0; i < m; ++i) {
    scanf("%d%d", &a, &b);
    a--;
    b--;
    adj[a].push_back(b);
  }
  for (i = 0; i < n; ++i) {
    if (!used[i]) {
      topological_sort(i);
    }
  }
  if (flag) {
    printf("-1\n");
    return 0;
  }
  int ans = 0;
  for (i = 0; i < n; ++i) {
    for (j = 0; j < 26; ++j) {
      ans = max(ans, freq[i][j]);
    }
  }
  printf("%d\n", ans);
  return 0;
}
