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.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.util.Collection;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.Queue;
import java.util.ArrayDeque;
import java.io.InputStream;

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

    static class TaskD {
        int n;
        int m;
        char[] letter;
        List<Edge>[] g;
        List<Edge>[] gr;

        public void solve(int testNumber, InputReader in, OutputWriter out) {
            n = in.readInt();
            m = in.readInt();
            letter = in.next().toCharArray();
            g = new List[n];
            gr = new List[n];
            for (int i = 0; i < n; i++) {
                g[i] = new ArrayList<>();
                gr[i] = new ArrayList<>();
            }

            for (int i = 0; i < m; i++) {
                int u = in.readInt() - 1;
                int v = in.readInt() - 1;
                g[u].add(new Edge(u, v));
                gr[v].add(new Edge(v, u));
            }

            if (new Cycles().anyCycle()) {
                out.print(-1);
                return;
            }

            out.print(new Solver().solve());
        }

        class Solver {
            int solve() {
                int[] inDegree = new int[n];
                for (int i = 0; i < n; i++) {
                    for (Edge e : g[i]) {
                        inDegree[e.to]++;
                    }
                }
                Queue<Integer> q = new ArrayDeque<>();
                for (int i = 0; i < n; i++) {
                    if (inDegree[i] == 0) {
                        q.add(i);
                    }
                }
                int[][] answer = new int[n][26];
                while (!q.isEmpty()) {
                    int cur = q.poll();
                    for (Edge e : gr[cur]) {
                        for (char c = 0; c < 26; c++) {
                            answer[cur][c] = Math.max(answer[cur][c], answer[e.to][c]);
                        }
                    }
                    answer[cur][letter[cur] - 'a']++;
                    for (Edge e : g[cur]) {
                        if (--inDegree[e.to] == 0) {
                            q.add(e.to);
                        }
                    }
                }
                int mx = answer[0][0];
                for (int[] row : answer) {
                    for (int i : row) {
                        mx = Math.max(mx, i);
                    }
                }
                return mx;
            }

        }

        class Cycles {
            int[] color;

            Cycles() {
                color = new int[n];
            }

            boolean dfs(int v) {
                color[v] = 1;
                for (Edge e : g[v]) {
                    if (color[e.to] == 0) {
                        if (dfs(e.to)) {
                            return true;
                        }
                    } else if (color[e.to] == 1) {
                        return true;
                    }
                }
                color[v] = 2;
                return false;
            }

            boolean anyCycle() {
                for (int i = 0; i < n; i++) {
                    if (color[i] == 0) {
                        if (dfs(i)) {
                            return true;
                        }
                    }
                }
                return false;
            }

        }

        class Edge {
            int from;
            int to;

            public Edge(int from, int to) {
                this.from = from;
                this.to = to;
            }

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void close() {
            writer.close();
        }

        public void print(int i) {
            writer.print(i);
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int readInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                if (Character.isValidCodePoint(c)) {
                    res.appendCodePoint(c);
                }
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next() {
            return readString();
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }
}

