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.Set;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
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 prakharjain
 */
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);
        DSubstring solver = new DSubstring();
        solver.solve(1, in, out);
        out.close();
    }

    static class DSubstring {
        public void solve(int testNumber, InputReader in, OutputWriter out) {

            int n = in.nextInt();

            int m = in.nextInt();

            String s = in.next();

            List[] g = new List[n];

            in.readDirectedGraph(g, n, m, 1);

            CycleFinder cf = new CycleFinder(g, n);

            boolean is = cf.isCycleDirected();

            if (is) {
                out.println(-1);
                return;
            }

            int[][] dp = new int[n][26];

            Set[] to = new Set[n];

            int[] ind = new int[n];

            for (int i = 0; i < n; i++) {
                to[i] = new HashSet();
            }
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < g[i].size(); j++) {
                    int v = (int) g[i].get(j);

                    to[v].add(i);

                    ind[v]++;
                }

            }

            int le = -1;
            ArrayDeque<Integer> q = new ArrayDeque<>();
            for (int i = 0; i < n; i++) {
                if (ind[i] == 0) {
                    q.addLast(i);
                    dp[i][s.charAt(i) - 'a'] = 1;
                }
            }

            int ans = 1;

            while (!q.isEmpty()) {
                int i = q.removeFirst();

                for (int j = 0; j < 26; j++) {
                    ans = Math.max(ans, dp[i][j]);
                }

                for (int j = 0; j < g[i].size(); j++) {
                    int v = (int) g[i].get(j);

                    for (int k = 0; k < 26; k++) {
                        dp[v][k] = Math.max(dp[v][k], dp[i][k]);
                    }

                    int ch = s.charAt(v) - 'a';
                    dp[v][ch] = Math.max(dp[v][ch], dp[i][ch] + 1);

                    ind[v]--;
                    if (ind[v] == 0) {
                        q.addLast(v);
                    }
                }
            }

            out.println(ans);
        }

        class CycleFinder {
            List[] g;
            int n;
            boolean isCycle;
            boolean[] recstack;
            boolean[] vis;

            public CycleFinder(List[] g, int n) {
                this.g = g;
                this.n = n;
                this.recstack = new boolean[n];
                this.vis = new boolean[n];
            }

            boolean isCycleDirected() {
                isCycle = false;

                for (int i = 0; i < n; i++) {
                    if (!vis[i]) {
                        dfs(i);
                    }
                }

                return isCycle;
            }

            private void dfs(int u) {
                recstack[u] = true;

                vis[u] = true;

                for (int v : (List<Integer>) g[u]) {
                    if (recstack[v]) {
                        isCycle = true;
                    }
                    if (!vis[v]) {
                        dfs(v);
                    }
                }

                recstack[u] = false;
            }

        }

    }

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

        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 String nextString() {
            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 String next() {
            return nextString();
        }

        public void readDirectedGraph(List[] g, int numOfVertices, int numOfEdges, int offset) {
            for (int i = 0; i < numOfVertices; i++) {
                g[i] = new ArrayList();
            }

            for (int i = 0; i < numOfEdges; i++) {
                int u = nextInt() - offset;
                int v = nextInt() - offset;

                g[u].add(v);
            }
        }

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

        }

    }
}

