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.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.ArrayDeque;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author gaidash
 */
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 {
        private void addEdge(Vertex from, Vertex to) {
            from.outgo.add(new Edge(to));
        }

        public void solve(int testNumber, InputReader in, OutputWriter out) {
            final int LETTERS = 26;

            int nVertices = in.nextInt();
            int nEdges = in.nextInt();
            Vertex[] vs = new Vertex[nVertices + 1];
            for (int i = 1; i <= nVertices; i++) {
                vs[i] = new Vertex(i, in.nextCharacter());
            }
            for (int i = 0; i < nEdges; i++) {
                int from = in.nextInt();
                int to = in.nextInt();
                vs[to].inDegree++;
                addEdge(vs[from], vs[to]);
            }

            int[][] dp = new int[LETTERS][nVertices + 1];
            ArrayDeque<Vertex> q = new ArrayDeque<>();
            for (int i = 1; i <= nVertices; i++) {
                if (vs[i].inDegree == 0) {
                    q.addLast(vs[i]);
                    dp[vs[i].val - 'a'][i] = 1;
                }
            }

            int count = 0;
            while (!q.isEmpty()) {
                Vertex cur = q.removeFirst();
                for (Edge e : cur.outgo) {
                    Vertex next = e.dest;
                    for (int i = 0; i < LETTERS; i++) {
                        dp[i][next.index] = Math.max(dp[i][next.index], dp[i][cur.index] + (next.val - 'a' == i ? 1 : 0));
                    }
                    next.inDegree--;
                    if (next.inDegree == 0) q.addLast(next);
                }
                count++;
            }

            if (count != nVertices) {
                out.print(-1);
                return;
            }

            int ret = 0;
            for (int i = 0; i < LETTERS; i++) {
                for (int j = 1; j <= nVertices; j++) {
                    ret = Math.max(ret, dp[i][j]);
                }
            }
            out.print(ret);
        }

        private class Vertex {
            private int index;
            private char val;
            private int inDegree = 0;
            private ArrayList<Edge> outgo = new ArrayList<>();

            private Vertex(int index, char val) {
                this.index = index;
                this.val = val;
            }

            public String toString() {
                StringBuilder ret = new StringBuilder("(" + index + ", " + val + ", " + inDegree + ") Goes to:");
                for (Edge edge : outgo) {
                    ret.append("(").append(edge.dest.index).append(", ").append(edge.dest.val).append(", ")
                            .append(edge.dest.inDegree).append(") ");
                }
                return ret.toString();
            }

        }

        private class Edge {
            private Vertex dest;

            private Edge(Vertex dest) {
                this.dest = dest;
            }

        }

    }

    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 nextInt() {
            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 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 char nextCharacter() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            return (char) c;
        }

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

        }

    }

    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);
        }

    }
}

