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.InputMismatchException;
import java.io.IOException;
import java.util.Stack;
import java.util.ArrayList;
import java.util.List;
import java.util.Vector;
import java.io.InputStream;

/**
 * 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 {
        static boolean[] inc;
        static boolean[] visited;
        static boolean[][] yes;
        static List<Integer>[] g;
        static long oo = Long.MAX_VALUE >> 2;
        static long ans = 0;
        static int[][] ct;
        static char[] s;

        public void solve(int testNumber, FastReader in, PrintWriter out) {
            int n = in.nextInt();
            int m = in.nextInt();
            //input
            ans = 0;
            s = in.nextString().toCharArray();
            g = GraphUtils.nextD(in, n, m, true);
            visited = new boolean[n];
            inc = new boolean[n];
            yes = new boolean[n][26];
            ct = new int[n][26];

            //proc
            for (int i = 0; i < n; ++i) ct[i][s[i] - 'a']++;
            for (int i = 0; i < n; ++i) yes[i][s[i] - 'a'] = true;
            for (int i = 0; i < n; ++i) if (!visited[i]) dfs(i);
            System.gc();
            List<List<Integer>> scc = new StronglyConnectedComponents(g).getComponents();

            //ans
            boolean flag = ans == oo;
            for (List<Integer> comp : scc)
                flag |= comp.size() > 1;
            if (flag) {
                out.println("-1");
            } else {
                int max = 0;
                for (int i = 0; i < n; ++i) {
                    for (int j = 0; j < 26; ++j) {
                        max = Math.max(max, ct[i][j]);
                    }
                }
                out.println(max);
            }
        }

        private static void dfs(int cur) {
            if (ans == oo) return;
            visited[cur] = true;
            inc[cur] = true;
            for (Integer next : g[cur]) {
                if (inc[next]) {
                    ans = oo;
                    return;
                }
                if (!visited[next]) dfs(next);
                for (int i = 0; i < 26; ++i) {
                    ct[cur][i] = Math.max(ct[cur][i], ct[next][i] + (yes[cur][i] ? 1 : 0));
                }
            }
            inc[cur] = false;
        }

    }

    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;
        }

    }

    static class StronglyConnectedComponents {
        private List<Integer>[] graph;
        private boolean[] visited;
        private Stack<Integer> stack;
        private int time;
        private int[] lowlink;
        private List<List<Integer>> components;

        public List<List<Integer>> getComponents() {
            return components;
        }

        public StronglyConnectedComponents(List<Integer>[] g) {
            this.graph = g;
            components = scc();
        }

        public List<List<Integer>> scc() {
            int n = graph.length;
            visited = new boolean[n];
            stack = new Stack<>();
            time = 0;
            lowlink = new int[n];
            components = new ArrayList<>();

            for (int u = 0; u < n; u++)
                if (!visited[u])
                    dfs(u);

            return components;
        }

        void dfs(int u) {
            lowlink[u] = time++;
            visited[u] = true;
            stack.add(u);
            boolean isComponentRoot = true;

            for (int v : graph[u]) {
                if (!visited[v])
                    dfs(v);
                if (lowlink[u] > lowlink[v]) {
                    lowlink[u] = lowlink[v];
                    isComponentRoot = false;
                }
            }

            if (isComponentRoot) {
                List<Integer> component = new ArrayList<>();
                while (true) {
                    int x = stack.pop();
                    component.add(x);
                    lowlink[x] = Integer.MAX_VALUE;
                    if (x == u)
                        break;
                }
                components.add(component);
            }
        }

    }

    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;
        }

    }
}

