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.*;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.*;

public class Solution {
    static StringBuffer str = new StringBuffer();
    static InputReader in = new InputReader(System.in);
    private final int V;
    private final List<List<Integer>> adj;
 
    public Solution(int V) 
    {
        this.V = V;
        adj = new ArrayList<>(V);
         
        for (int i = 0; i < V; i++)
            adj.add(new LinkedList<>());
    }
    private boolean isCyclicUtil(int i, boolean[] visited,
                                      boolean[] recStack) 
    {
        if (recStack[i])
            return true;
 
        if (visited[i])
            return false;
             
        visited[i] = true;
 
        recStack[i] = true;
        List<Integer> children = adj.get(i);
         
        for (Integer c: children)
            if (isCyclicUtil(c, visited, recStack))
                return true;
                 
        recStack[i] = false;
 
        return false;
    }
 
    private void addEdge(int source, int dest) {
        adj.get(source).add(dest);
    }
    private boolean isCyclic() 
    {
        boolean[] visited = new boolean[V];
        boolean[] recStack = new boolean[V];
        for (int i = 0; i < V; i++)
            if (isCyclicUtil(i, visited, recStack))
                return true;
 
        return false;
    }
    ArrayList<Integer> graph[];
    boolean visited[];
    int freq[][];
    String s;
    public void dfs(int x){
        visited[x]=true;
        freq[x][s.charAt(x)-97]=1;
        int zz[]=new int[26];
        zz[s.charAt(x)-97]=1;
        for(Integer it:graph[x]){
            if(!visited[it]){
                dfs(it);
                int pp[]=new int[26];
                for(int i=0;i<26;i++){
                    pp[i]=freq[x][i]+freq[it][i];
                    zz[i]=Math.max(pp[i], zz[i]);
                }                
            }else{
                int pp[]=new int[26];
                for(int i=0;i<26;i++){
                    pp[i]=freq[x][i]+freq[it][i];
                    zz[i]=Math.max(pp[i], zz[i]);
                }
            }
        }
        for(int i=0;i<26;i++){
            freq[x][i]=zz[i];
        }
    }
    public void callme(int n,int m){
        int i,j;
        graph=new ArrayList[n];
        visited=new boolean[n];
        for(i=0;i<n;i++){
            graph[i]=new ArrayList<>();
        }
        freq=new int[n][26];
        s=in.next();
        for(i=0;i<m;i++){
            int x=in.nextInt()-1;
            int y=in.nextInt()-1;
            addEdge(x,y);
            graph[x].add(y);
        }
        if(isCyclic()){
            System.out.println(-1);
        }else{
            for(i=0;i<n;i++){
                if(!visited[i])
                    dfs(i);
            }
            int max=0;
            for(i=0;i<n;i++){
                for(j=0;j<26;j++){
                    max=Math.max(max,freq[i][j]);
                }
            }
            System.out.println(max);
        }
    }
    public static void main(String[] args) {
        int t,n,m,i,j;
        n=in.nextInt();
        m=in.nextInt();
        Solution g=new Solution(n);
        g.callme(n, m);
    }
    public static void ap(String st){
        str.append(st);
    }
    public static void pn() {
        System.out.println(str);
    }
    static class InputReader {

        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;

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

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

        public int peek() {
            if (numChars == -1) {
                return -1;
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    return -1;
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar];
        }

        public void skip(int x) {
            while (x-- > 0) {
                read();
            }
        }

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

        public long nextLong() {
            return Long.parseLong(next());
        }

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

        public String next() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuffer res = new StringBuffer();
            do {
                res.appendCodePoint(c);
                c = read();
            } while (!isSpaceChar(c));

            return res.toString();
        }

        public String nextLine() {
            StringBuffer buf = new StringBuffer();
            int c = read();
            while (c != '\n' && c != -1) {
                if (c != '\r') {
                    buf.appendCodePoint(c);
                }
                c = read();
            }
            return buf.toString();
        }

        public double nextDouble() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') {
                if (c == 'e' || c == 'E') {
                    return res * Math.pow(10, nextInt());
                }
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) {
                    if (c == 'e' || c == 'E') {
                        return res * Math.pow(10, nextInt());
                    }
                    if (c < '0' || c > '9') {
                        throw new InputMismatchException();
                    }
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }

        public boolean hasNext() {
            int value;
            while (isSpaceChar(value = peek()) && value != -1) {
                read();
            }
            return value != -1;
        }

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