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.lang.reflect.Array;
import java.util.*;

public class TaskD {
    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);
        Solve solve = new Solve();
        solve.solve(in, out);
        out.close();
    }


    private static class Solve {
        int n, m;
        ArrayList<ArrayList<Integer>> v = new ArrayList<ArrayList<Integer>>();
        ArrayList<ArrayList<Integer>> g = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> topsort = new ArrayList<Integer>();
        char[] s;
        int[] used;
        boolean isBad;
        int[][] cnt;
        int[] f;

        public void solve(InputReader in, PrintWriter out) {
            n = in.nextInt();
            used = new int[n];
            f = new int[n];
            cnt = new int[n][26];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < 26; j++) {
                    cnt[i][j] = -1;
                }
            }
            m = in.nextInt();
            s = in.next().toCharArray();
            for (int i = 0; i < n; i++) {
                v.add(new ArrayList<Integer>());
                g.add(new ArrayList<Integer>());
            }
            for (int i = 0; i < m; i++) {
                int q = in.nextInt();
                int w = in.nextInt();
                q--;
                w--;
                f[q]++;
                g.get(q).add(w);
                v.get(w).add(q);
            }
            isBad = false;
            for (int i = 0; i < n; i++) {
                if (used[i] == 0) {
                    dfs(i);
                }
            }
            if (isBad) {
                out.print(-1);
                return;
            }
            used = new int[n];
            ArrayList<Integer> st;
            for (int i = 0; i < n; i++) {
                int vertex = topsort.get(n-i-1);
                if (used[vertex] == 0) {
                    used[vertex] = 1;
                    add(vertex, s[vertex], true);
                }
                for (int newVer : v.get(vertex)) {
                    if (better(vertex, newVer)) {
                        used[newVer] = 1;
                    }
                }
            }
//            for (int i = 0; i < n; i++) {
//                for (int j = 0; j < 26; j++) {
//                    out.print(cnt[i][j] + " ");
//                }
//                out.println();
//            }
            int answer = 0;
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < 26; j++) {
                    answer = StrictMath.max(answer, cnt[i][j]);
                }
            }
            out.print(answer);
        }

        private boolean better(int ver, int newVer) {
            boolean ret = false;
            for (int i = 0; i < 26; i++) {
                if (cnt[ver][i] + foo(newVer, i) > cnt[newVer][i]) {
                    cnt[newVer][i] = cnt[ver][i] + foo(newVer, i);
                    ret = true;
                }
            }
            return ret;
        }

        private int foo(int x, int i) {
            return (s[x] - 'a' == i ? 1 : 0);
        }

        private void add(int ind, char c, boolean isNew) {
            int x = c - 'a';
            if (isNew) {
                for (int i = 0; i < 26; i++) {
                    cnt[ind][i] = 0;
                }
            }
            cnt[ind][x]++;
        }

        private void dfs(int x) {
            used[x] = 1;
            for (int next : v.get(x)) {
                if (used[next] == 0) {
                    dfs(next);
                } else {
                    if (used[next] == 1) {
                        isBad = true;
                    }
                }
            }
            used[x] = 2;
            topsort.add(x);
        }

        private class Pair {
            int a;
            int b;

            public Pair(int a, int b) {
                this.a = a;
                this.b = b;
            }
        }
    }

    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());
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

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