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.util.*;
import java.io.*;

public class Solution1 {

    private void solve() throws IOException {
        int n = in.nextInt();
        int m = in.nextInt();
        List<List<Integer>> g = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            g.add(new ArrayList<>());
        }
        String s = in.next();
        int[] cnt_edge = new int[n];
        for (int i = 0; i < m; i++) {
            int x = in.nextInt() - 1;
            int y = in.nextInt() - 1;
            if (x == y) {
                System.out.println(-1);
                return;
            }
            g.get(x).add(y);
            cnt_edge[y]++;
        }
        int[] color = new int[n];
        for (int i = 0; i < n; i++) {
            if (color[i] == 0) {
                if (dfs_cic(g, i, color)) {
                    System.out.println(-1);
                    return;
                }
            }
        }
        int[][] dp = new int[n][26];
        Queue<Integer> q = new ArrayDeque<>();
        for (int i = 0; i < n; i++) {
            if (cnt_edge[i] == 0) {
                q.add(i);
                dp[i][s.charAt(i) - 'a']++;
            }
        }

        while (!q.isEmpty()) {
            int f = q.poll();
            for (int i : g.get(f)) {
                for (int j = 0; j < 26; j++) {
                    dp[i][j] = Math.max(dp[i][j], dp[f][j] + ((s.charAt(i) - 'a') == j ? 1 : 0));
                }
                cnt_edge[i]--;
                if (cnt_edge[i] == 0)
                    q.add(i);
            }
        }
        
        int ans = Integer.MIN_VALUE;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < 26; j++) {
                ans = Math.max(ans, dp[i][j]);
            }
        }
        System.out.println(ans);
    }

    private boolean dfs_cic(List<List<Integer>> g, int k, int[] color) {
        color[k] = 1;
        for (int i : g.get(k)) {
            if (color[i] == 0) {
                if (dfs_cic(g, i, color))
                    return true;
            }
            if (color[i] == 1) {
                return true;
            }
        }
        color[k] = 2;
        return false;
    }



    private String filename = "";
    private PrintWriter out;
    MyScanner in;

    private void run() throws IOException {
        in = new MyScanner();
        out = new PrintWriter(System.out);
        solve();
        in.close();
        out.close();
    }

    private class MyScanner {
        private BufferedReader br;
        private StringTokenizer st;

        public MyScanner() throws IOException {
            this.br = new BufferedReader(new InputStreamReader(System.in));
        }

        public MyScanner(String fileTitle) throws IOException {
            this.br = new BufferedReader(new FileReader(fileTitle));
        }

        public String nextLine() throws IOException {
            String s = br.readLine();
            return s == null ? "-1" : s;
        }

        public String next() throws IOException {
            while (st == null || !st.hasMoreTokens()) {
                String s = br.readLine();
                if (s == null) {
                    return "-1";
                }
                st = new StringTokenizer(s);
            }
            return st.nextToken();
        }

        public Integer nextInt() throws IOException {
            return Integer.parseInt(this.next());
        }

        public Long nextLong() throws IOException {
            return Long.parseLong(this.next());
        }

        public Double nextDouble() throws IOException {
            return Double.parseDouble(this.next());
        }

        public void close() throws IOException {
            this.br.close();
        }
    }

    public static void main(String[] args) throws IOException {
        Locale.setDefault(Locale.US);
        new Solution1().run();
    }
}
