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.Collection;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;
import java.io.BufferedReader;
import java.util.Queue;
import java.io.FileReader;
import java.util.LinkedList;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author ShekharN
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastScanner in = new FastScanner(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, FastScanner in, PrintWriter out) {
            int n = in.nextInt(), m = in.nextInt();
            String s = in.nextString();
            List<Integer>[] graph = new List[n + 1];
            for (int i = 1; i <= n; i++) {
                graph[i] = new ArrayList<>();
            }
            int[] indeg = new int[n + 1];

            for (int i = 0; i < m; i++) {
                int x = in.nextInt(), y = in.nextInt();
                graph[x].add(y);
                indeg[y]++;
            }

            int ans = topoSort(graph, indeg, n, s);
            out.println(ans);
        }

        private int topoSort(List<Integer>[] graph, int[] indeg, int n, String vals) {
            int[][] freq = new int[n + 1][26];
            int[] vis = new int[n + 1];
            Queue<Integer> queue = new LinkedList<>();
            for (int i = 1; i <= n; i++) {
                if (indeg[i] == 0) {
                    queue.add(i);
                    freq[i][vals.charAt(i - 1) - 'a']++;
                }
            }
            while (!queue.isEmpty()) {
                int cur = queue.remove();
                vis[cur] = 1;
                for (int i = 0; i < graph[cur].size(); i++) {
                    int v = graph[cur].get(i);
                    indeg[v]--;
                    if (indeg[v] == 0)
                        queue.add(v);
                    for (int j = 0; j < 26; j++) {
                        if (vals.charAt(v - 1) - 'a' == j) {
                            freq[v][j] = Math.max(freq[v][j], freq[cur][j] + 1);
                        } else {
                            freq[v][j] = Math.max(freq[v][j], freq[cur][j]);
                        }
                    }
                }
            }
            for (int i = 1; i <= n; i++) {
                if (vis[i] == 0)
                    return -1;
            }
            int mx = -1;
            for (int i = 1; i <= n; i++) {
                for (int j = 0; j < 26; j++) {
                    mx = Math.max(mx, freq[i][j]);
                }
            }
            return mx;
        }

    }

    static class FastScanner {
        private BufferedReader br;
        private StringTokenizer st;

        public FastScanner(File f) {
            try {
                br = new BufferedReader(new FileReader(f));
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            }
        }

        public FastScanner(InputStream f) {
            br = new BufferedReader(new InputStreamReader(f));
        }

        public String nextString() {
            while (st == null || !st.hasMoreTokens()) {
                String s = null;
                try {
                    s = br.readLine();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (s == null)
                    return null;
                st = new StringTokenizer(s);
            }
            return st.nextToken();
        }

        public int nextInt() {
            return Integer.parseInt(nextString());
        }

    }
}

