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.*;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;

public class D {

    FastScanner scanner;
    PrintWriter writer;

    List<List<Integer>> g = new ArrayList<>();
    int[][] vals;
    String l;

    void solve() throws IOException {
        scanner = new FastScanner(System.in);
        try (PrintWriter writer = new PrintWriter(System.out)) {

            int n = scanner.nextInt();
            int m = scanner.nextInt();
            l = scanner.nextLine();

            for (int i = 0; i < n; i++) {
                g.add(new ArrayList<>());
            }

            for (int i = 0; i < m; i++) {
                int a = scanner.nextInt() - 1;
                int b = scanner.nextInt() - 1;

                g.get(a).add(b);
            }

            vals = new int[n][26];

            int[] col = new int[n];

            for (int i = 0; i < n; i++) {
                if (col[i] == 0)
                    if (calcVals(i, col)) {
                        writer.println(-1);
                        return;
                    }
            }

            int max = 0;

            for (int i = 0; i < n; i++) {
                for (int j = 0; j < 26; j++) {
                    max = Math.max(max, vals[i][j]);
                }
            }

            writer.println(max);
        }
    }

    boolean calcVals(int v, int[] col) {
        col[v] = 1;

        int x = l.charAt(v) - 'a';

        vals[v][x] = 1;

        for (int e : g.get(v)) {
            if (col[e] == 1) {
                return true;
            } else if (col[e] == 0) {
                if (calcVals(e, col)) {
                    return true;
                }
            }

            for (int i = 0; i < 26; i++) {
                int t = vals[e][i];
                if (i == x)
                    t++;

                vals[v][i] = Math.max(vals[v][i], t);
            }
        }

        col[v] = 2;

        return false;
    }

    public static void main(String... args) throws IOException {
        new D().solve();
    }

    static class FastScanner {

        BufferedReader br;
        StringTokenizer tokenizer;

        FastScanner(String fileName) throws FileNotFoundException {
            this(new FileInputStream(new File(fileName)));
        }

        FastScanner(InputStream is) {
            br = new BufferedReader(new InputStreamReader(is));
        }

        String nextLine() throws IOException {
            tokenizer = null;
            return br.readLine();
        }

        String next() throws IOException {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                String line = br.readLine();
                if (line == null) {
                    return null;
                }
                tokenizer = new StringTokenizer(line);
            }
            return tokenizer.nextToken();
        }

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

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