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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author rizhiy
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskD solver = new TaskD();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskD {
        ArrayList<Integer>[] g;
        int[] used;
        char[] a;
        int[][] dp;

        int dfs(int v) {
            used[v] = 1;
            for (int i = 0; i < g[v].size(); i++) {
                int to = g[v].get(i);
                if (used[to] == 1) {
                    return -1;
                }
                if (used[to] == 0) {
                    if (dfs(to) == -1)
                        return -1;
                }
                for (int j = 0; j < 30; j++) {
                    dp[v][j] = Math.max(dp[v][j], dp[to][j]);
                }
            }
            dp[v][a[v] - 'a']++;
            used[v] = 2;
            return 0;
        }

        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            a = in.next().toCharArray();
            g = new ArrayList[n];
            dp = new int[n][30];
            used = new int[n];
            for (int i = 0; i < n; i++) {
                g[i] = new ArrayList<>();
                used[i] = 0;
            }
            for (int i = 0; i < m; i++) {
                int x = in.nextInt();
                int y = in.nextInt();
                x--;
                y--;
                g[x].add(y);
            }
            for (int i = 0; i < n; i++) {
                if (used[i] == 0) {
                    if (dfs(i) == -1) {
                        out.println(-1);
                        return;
                    }
                }
            }
            int mav = 0;
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < 30; j++) {
                    mav = Math.max(mav, dp[i][j]);
                }
            }
            out.println(mav);
        }

    }

    static class InputReader {
        public BufferedReader reader;
        public StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

    }
}

