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.InputStreamReader;
import java.util.ArrayList;
import java.util.Collection;

@SuppressWarnings("unchecked")
public class D {

    private static char[] s;
    private static Collection<Integer>[] graph;

    private static int[] visited;
    private static int[][] max;
    private static boolean cycle;

    private static void dfs(int v) {
        visited[v] = 1;

        for (int w : graph[v]) {
            if (visited[w] == 1)
                cycle = true;
            else {
                if (visited[w] == 0)
                    dfs(w);
                for (int i = 0; i < 26; i++)
                    max[v][i] = Math.max(max[v][i], max[w][i]);
            }
        }

        max[v][s[v] - 'a']++;
        visited[v] = 2;
    }

    public static void main(String[] args) throws Exception {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String[] splits;

        splits = in.readLine().split(" ");
        int n = Integer.parseInt(splits[0]);
        int m = Integer.parseInt(splits[1]);

        s = in.readLine().toCharArray();

        graph = (Collection<Integer>[]) new Collection[n];
        for (int i = 0; i < n; i++)
            graph[i] = new ArrayList<>();

        while (m-- > 0) {
            splits = in.readLine().split(" ");
            graph[Integer.parseInt(splits[0]) - 1].add(Integer.parseInt(splits[1]) - 1);
        }

        visited = new int[n];
        max = new int[n][26];

        for (int v = 0; v < n; v++) {
            if (visited[v] == 0)
                dfs(v);
        }

        if (cycle) {
            System.out.println(-1);
            return;
        }

        int max = 0;
        for (int v = 0; v < n; v++) {
            for (int i = 0; i < 26; i++)
                max = Math.max(max, D.max[v][i]);
        }
        System.out.println(max);
    }

}
