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.PrintWriter;
import java.util.List;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;

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

    static class TaskD {
        public void solve(int testNumber, FastReader in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            char[] s = in.nextString().toCharArray();
            List<Integer>[] g = GraphUtils.nextD(in, n, m, true);
            int[] ord = GraphUtils.sortTopologically(g);
            if (ord == null) {
                out.println(-1);
                return;
            }
            int[][] dp = new int[n][26];
            for (int i = 0; i < n; ++i) {
                int cur = ord[i];
                ++dp[cur][s[cur] - 'a'];
                for (Integer next : g[cur]) {
                    for (int j = 0; j < 26; ++j) dp[next][j] = Math.max(dp[next][j], dp[cur][j]);
                }
            }
            int ans = 0;
            for (int[] ddp : dp)
                for (int dddp : ddp)
                    ans = Math.max(ans, dddp);
            out.println(ans);
        }

    }

    static class GraphUtils {
        public static List<Integer>[] nextD(FastReader in, int n, int m, boolean oneIndexed) {
            List<Integer>[] g = new List[n];
            for (int i = 0; i < n; ++i) g[i] = new ArrayList<>();
            for (int i = 0; i < m; ++i) {
                int u = in.nextInt() - (oneIndexed ? 1 : 0);
                int v = in.nextInt() - (oneIndexed ? 1 : 0);
                g[u].add(v);
            }
            return g;
        }

        public static int[] sortTopologically(List<Integer>[] g) {
            int n = g.length;
            int[] indeg = new int[n];
            for (int cur = 0; cur < n; ++cur)
                for (Integer next : g[cur])
                    ++indeg[next];
            int[] ret = new int[n];
            int ptr = 0;

            //sources
            for (int i = 0; i < n; ++i)
                if (indeg[i] == 0) ret[ptr++] = i;

            //remaining nodes
            for (int start = 0; start < ptr; ++start)
                for (Integer next : g[ret[start]])
                    if (--indeg[next] == 0) ret[ptr++] = next;

            //loop
            for (int i = 0; i < n; ++i)
                if (indeg[i] > 0) return null;
            return ret;
        }

    }

    static class FastReader {
        private InputStream stream;
        private byte[] buf = new byte[8192];
        private int curChar;
        private int pnumChars;

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

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

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

        public String nextString() {
            int c = pread();
            while (isSpaceChar(c))
                c = pread();
            StringBuilder res = new StringBuilder();
            do {
                res.appendCodePoint(c);
                c = pread();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        private boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

    }
}

