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;
long long int n, m;
string s;
long long int x, y;
vector<long long int> edges[300005];
long long int indegree[300005];
vector<long long int> toposort;
long long int dp[300005];
void solve(int tc = 0) {
  cin >> n >> m >> s;
  for (long long int i = 0; i < m; i++) {
    cin >> x >> y;
    --x;
    --y;
    edges[x].push_back(y);
    ++indegree[y];
  }
  queue<long long int> q;
  long long int cnt = 0;
  for (long long int i = 0; i < n; i++) {
    if (indegree[i] == 0) {
      q.push(i);
    }
  }
  while (!q.empty()) {
    ++cnt;
    long long int x = q.front();
    q.pop();
    toposort.push_back(x);
    for (long long int y : edges[x]) {
      --indegree[y];
      if (indegree[y] == 0) {
        q.push(y);
      }
    }
  }
  if (cnt < n) {
    cout << -1 << '\n';
    return;
  }
  long long int ans = 0;
  for (char c = 'a'; c <= 'z'; c++) {
    memset(dp, 0, sizeof(dp));
    for (long long int x : toposort) {
      if (s[x] == c) ++dp[x];
      for (long long int y : edges[x]) {
        dp[y] = max(dp[y], dp[x]);
      }
      ans = max(ans, dp[x]);
    }
  }
  cout << ans << '\n';
}
int main() {
  int tc = 1;
  for (int t = 0; t < tc; t++) solve(t);
}
