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.*;


/**
 * Created by Katushka on 08.02.2020.
 */
public class Substring {

    static class Node {
        char letter;
        int index;
        int[] maxOccs = new int[26];

        List<Node> children = new ArrayList<>();
        boolean hasParents = false;

        public Node(char letter, int index) {
            this.letter = letter;
            this.index = index;

            maxOccs[letter - 'a'] = 1;
        }
    }

    public static void main(String[] args) throws FileNotFoundException {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));

        int n = in.nextInt();
        int m = in.nextInt();
        String s = in.nextString();

        List<Node> nodes = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            char ch = s.charAt(i);
            nodes.add(new Node(ch, i));
        }
        for (int i = 0; i < m; i++) {
            int v1 = in.nextInt();
            int v2 = in.nextInt();

            nodes.get(v1 - 1).children.add(nodes.get(v2 - 1));
            nodes.get(v2 - 1).hasParents = true;

        }

        Collection<Node> starts = new HashSet<>();
        for (Node node : nodes) {
            if (!node.hasParents) {
                starts.add(node);
            }
        }
        if (starts.isEmpty()) {
            out.println(-1);
        } else {
            Set<Node> visited = new HashSet<>();
            Set<Node> path = new HashSet<>();
            for (Node node : starts) {
                if (!fill(node, visited, path)) {
                    out.println(-1);
                    out.close();
                    return;
                }
            }
            if (visited.size() != n) {
                out.println(-1);
            } else {
                int max = 0;
                for (Node node : nodes) {
                    for (int maxOcc : node.maxOccs) {
                        max = Math.max(max, maxOcc);
                    }
                }
                out.println(max);
            }
        }

        out.close();
    }

    private static boolean fill(Node node, Set<Node> visited, Set<Node> path) {
        visited.add(node);
        path.add(node);

        for (Node child : node.children) {
            if (path.contains(child)) {
                return false;
            }
            if (!visited.contains(child)) {
                if (!fill(child, visited, path)) {
                    return false;
                }
                path.remove(child);
            }

        }
        for (char ch = 'a'; ch <= 'z'; ch++) {
            for (Node child : node.children) {
                if (ch == node.letter) {
                    node.maxOccs[ch- 'a'] = Math.max(node.maxOccs[ch- 'a'], 1 + child.maxOccs[ch - 'a']);
                } else {
                    node.maxOccs[ch- 'a'] = Math.max(node.maxOccs[ch- 'a'], child.maxOccs[ch - 'a']);
                }
            }
        }
        return true;

    }

    private 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 String nextString() {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

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

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

        public char nextChar() {
            return next().charAt(0);
        }
    }
}
