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;
struct edge {
  int from, to;
  long long cost;
};
int n, m;
vector<int> G[300001];
char s[300001];
int gindeg[300001];
vector<int> topological_sort() {
  set<int> st;
  int indeg[n];
  fill(indeg, indeg + n, 0);
  for (int i = 0; i < n; i++)
    for (int to : G[i]) indeg[to]++;
  for (int i = 0; i < n; i++) {
    gindeg[i] = indeg[i];
    if (indeg[i] == 0) st.insert(i);
  }
  vector<int> ret;
  int used[n];
  fill(used, used + n, 0);
  while (!st.empty()) {
    int node = *st.begin();
    st.erase(node);
    ret.push_back(node);
    used[node] = 1;
    for (int next : G[node]) {
      indeg[next]--;
      if (!used[next] && indeg[next] == 0) st.insert(next);
    }
  }
  return ret;
}
int main() {
  int from, to, ans = -1;
  scanf("%d %d", &n, &m);
  if (n == 1) {
    cout << -1 << endl;
    return 0;
  }
  scanf("%s", s);
  for (int i = 0; i < m; i++) {
    scanf("%d %d", &from, &to);
    G[from - 1].push_back(to - 1);
  }
  vector<int> v = topological_sort();
  if ((int)v.size() < n) {
    cout << -1 << endl;
    return 0;
  }
  int dp[26][n];
  fill(dp[0], dp[26], 0);
  for (char c = 'a'; c <= 'z'; c++) {
    int ci = (int)(c - 'a');
    for (int i = 0; i < n; i++) {
      if (gindeg[v[i]] == 0) {
        dp[ci][v[i]] = (int)(s[v[i]] == c);
      }
      for (int next : G[v[i]]) {
        dp[ci][next] = max(dp[ci][next], dp[ci][v[i]] + (int)(s[next] == c));
      }
    }
    for (int i = 0; i < n; i++) ans = max(ans, dp[ci][i]);
  }
  printf("%d\n", ans);
  return 0;
}
