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 GCD(long long a, long long b) { return b ? GCD(b, a % b) : a; }
long long lcm(long long a, long long b) { return a * b / GCD(a, b); }
int lb(int x) { return x & (-x); }
const long long N = 3e5 + 7;
const long long inf = 1e18 + 1;
const long long mod2 = 998244353;
const long long mod1 = 1e9 + 7;
const int P1 = 13331;
const int P2 = 131;
const double eps = 1e-6;
const double dbinf = 1e13 + 7;
const long long mod3 = 10000 + 7;
int n, m;
string s;
struct node {
  int v, nxt;
};
node e[N];
int tot;
int head[N];
int ing[N];
int ans;
int dp[N][26];
void add(int u, int v) {
  e[++tot] = {v, head[u]};
  head[u] = tot;
}
void topsort(void) {
  queue<int> q;
  for (int i = 1; i <= n; i++) {
    if (ing[i] == 0) {
      q.push(i);
      dp[i][s[i] - 'a'] = 1;
    }
  }
  int cnt = 0;
  while (!q.empty()) {
    int x = q.front();
    cnt++;
    q.pop();
    for (int i = head[x]; i; i = e[i].nxt) {
      int v = e[i].v;
      if (--ing[v] == 0) q.push(v);
      for (int j = 0; j <= 25; j++) {
        dp[v][j] = max(dp[v][j], dp[x][j] + (s[v] - 'a' == j));
      }
    }
  }
  if (cnt != n) {
    cout << "-1\n";
  } else {
    int maxn = 0;
    for (int i = 1; i <= n; i++) {
      for (int j = 0; j <= 25; j++) {
        maxn = max(maxn, dp[i][j]);
      }
    }
    cout << maxn << "\n";
  }
}
int main() {
  memset(head, 0, sizeof head);
  memset(ing, 0, sizeof ing);
  cin >> n >> m;
  cin >> s;
  s = "*" + s;
  tot = 0;
  for (int i = 1, u = 1, v = 1; i <= m; i++) {
    cin >> u >> v;
    add(u, v);
    ing[v]++;
  }
  topsort();
  return 0;
}
