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 MAX = 3e5 + 6;
const int MOD = 1e9 + 7;
set<int> adj[MAX];
char str[MAX];
bool vis[MAX];
bool rec[MAX];
bool dfs(int u) {
  if (rec[u]) {
    return true;
  }
  if (vis[u]) return false;
  vis[u] = true;
  rec[u] = true;
  for (auto v : adj[u]) {
    if (dfs(v)) {
      return true;
    }
  }
  rec[u] = false;
  return false;
}
bool isLoop(int n) {
  for (int i = 0; i < n; i++) {
    if (vis[i]) continue;
    if (dfs(i)) {
      return true;
    }
  }
  return false;
}
int mat[MAX];
int findMaxPath(int u, int p, int x) {
  if (vis[u]) return mat[u];
  int res = 0;
  vis[u] = true;
  for (auto v : adj[u]) {
    auto tmp = findMaxPath(v, u, x);
    res = max(res, tmp);
  }
  res += (str[u] == x);
  mat[u] = res;
  return res;
}
int func(int n, int x) {
  memset(vis, 0, sizeof(vis));
  int res = 0;
  for (int i = 0; i < n; i++) {
    if (vis[i]) continue;
    res = max(res, findMaxPath(i, -1, x));
  }
  return res;
}
int main() {
  int n, m, u, v;
  scanf("%d%d", &n, &m);
  scanf("%s", str);
  for (int i = 0; i < m; i++) {
    scanf("%d%d", &u, &v);
    u--;
    v--;
    adj[u].insert(v);
  }
  if (isLoop(n)) {
    printf("-1\n");
    return 0;
  }
  int res = 0;
  for (int i = 'a'; i <= 'z'; i++) {
    res = max(res, func(n, i));
  }
  printf("%d\n", res);
  return 0;
}
