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.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class Solution {
    int ans = 0;
    int[] dfs(ArrayList<Integer>[] graph, int[] mark, int item,
                                  int[][] result, char[] colors) {
        if (mark[item]==1) {
            System.out.println(-1);
            System.exit(0);
        }
        if (mark[item]==2) return result[item];
        mark[item] = 1;
        int[] res = new int[27];
        for (int elem:graph[item]) {
            int[] arr = dfs(graph, mark, elem, result, colors);
            for (int i=0; i<27; i++) {
                res[i] = Math.max(res[i], arr[i]);
                ans = Math.max(ans, res[i]);
            }
        }
        int x = colors[item]-'a';
        res[x]++;
        ans = Math.max(ans, res[x]);
        mark[item] = 2;
        result[item] = res;
        return res;
    }
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer stoken = new StringTokenizer(reader.readLine());
        int n = Integer.parseInt(stoken.nextToken());
        int m = Integer.parseInt(stoken.nextToken());
        char[] colors = reader.readLine().toCharArray();
        ArrayList<Integer>[] graph = new ArrayList[n];
        for (int i=0; i<n; i++) graph[i] = new ArrayList<Integer>();
        Solution solve = new Solution();
        boolean infinity = false;
        for (int i=0; i<m; i++) {
            stoken = new StringTokenizer(reader.readLine());
            int x = Integer.parseInt(stoken.nextToken())-1;
            int y = Integer.parseInt(stoken.nextToken())-1;
            if (x==y) infinity = true;
            graph[x].add(y);
        }
        if (infinity) {
            System.out.println(-1);
            System.exit(0);
        }
        int[] mark = new int[n];
        int[][] result = new int[n][27];
        for (int i=0; i<n; i++) {
            if (mark[i]==0) {
                solve.dfs(graph, mark, i, result, colors);
            }
        }
        System.out.println(solve.ans);
    }
}