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.*;

public class a {
    public static void main(String[] args) throws IOException {
        FastReader scan = new FastReader();
        //PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("taming.out")));
        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        Task solver = new Task();
        //int t = scan.nextInt();
        int t = 1;
        for(int i = 1; i <= t; i++) solver.solve(i, scan, out);
        out.close();
    }
    static class Task {
    	int V;
        ArrayList<ArrayList<Integer>> adjl;
        public void solve(int testNumber, FastReader sc, PrintWriter pw) {
        	int n = sc.nextInt();
        	V=n+1;
        	int m = sc.nextInt();
        	int[] indeg = new int[n+1];
        	String str = sc.nextLine();
        	adjl = new ArrayList<>();
        	for(int i=0;i<=n;i++){
        		adjl.add(new ArrayList<>());
        	}
        	for(int i=0;i<m;i++){
        		int a = sc.nextInt();
        		int b = sc.nextInt();
        		indeg[b]++;
        		adjl.get(a).add(b);
        	}
        	if(isCyclic()){
        		pw.println(-1);return;
        	}
        	int ct = 1;
        	int[][] dp = new int[n+1][26];
        	int[] q = new int[5000000];
        	int ptr =0;
        	int po = 0;
        	for(int i=1;i<=n;i++){
        		if(indeg[i]==0){
        			q[ptr++]=i;
        			dp[i][str.charAt(i-1)-'a']++;
        		}
        	}
        	while(po!=ptr){
        		int t = q[po++];
        		for(int x : adjl.get(t)){
        			for(int i=0;i<26;i++){
        				dp[x][i]=Math.max(dp[t][i]+(str.charAt(x-1)=='a'+i?1:0),dp[x][i]);
        			}
        			if(indeg[x]>0){
        				indeg[x]--;
        			}
        			if(indeg[x]==0)
        				q[ptr++]=x;
        		}
        	}
        	int max = 0;
        	for(int[] x : dp){
        		for(int y : x){
        			max = Math.max(y,max);
        		}
        	}
        	pw.println(max);
        	
        	 
        }
        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; 
	        ArrayList<Integer> children = adjl.get(i); 
	        for (int c: children) 
	            if (isCyclicUtil(c, visited, recStack)) 
	                return true; 
	        recStack[i] = false; 
	        return false; 
	    } 
	    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; 
	    } 

    }
    static class tup implements Comparable<tup>{
        int a, b,c;
        tup(){};
        tup(int a, int b, int c){
            this.a=a;
            this.b=b;
            this.c=c;
        }

        @Override
        public int compareTo( tup o2) {
            return o2.b!=b?Integer.compare(o2.b,b):Integer.compare(o2.a,a);
        }
    }
    static void shuffle(long[] a) {
        Random get = new Random();
        for (int i = 0; i < a.length; i++) {
            int r = get.nextInt(a.length);
            long temp = a[i];
            a[i] = a[r];
            a[r] = temp;
        }
    }
    static void shuffle(int[] a) {
        Random get = new Random();
        for (int i = 0; i < a.length; i++) {
            int r = get.nextInt(a.length);
            int temp = a[i];
            a[i] = a[r];
            a[r] = temp;
        }
    }
    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        public FastReader(String s) throws FileNotFoundException {
            br = new BufferedReader(new FileReader(new File(s)));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

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

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

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }

}