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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Random;
import java.util.StringTokenizer;

public class Solution{
	
	

	public static void main(String[] args) {

		FastScanner fs = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		
		
		int tt = 1;
		while(tt-->0) {
			
			int n = fs.nextInt(), m = fs.nextInt();
			char[] c = fs.next().toCharArray();
			
			ArrayList<Integer>[] adjlist = new ArrayList[n];
			for(int i=0;i<n;i++) adjlist[i] = new ArrayList<Integer>();
			
			
			int[] deg = new int[n];
			int[][] dp = new int[n][26];
			
			for(int i=0;i<m;i++) {
				int u = fs.nextInt()-1, v = fs.nextInt()-1;
				adjlist[u].add(v);
				deg[v]++;
			}
			
			ArrayDeque<Integer> queue = new ArrayDeque<Integer>();
			
			for(int i=0;i<n;i++) {
				if(deg[i]==0) {
					queue.add(i);
					dp[i][c[i]-'a']++;
				}
			}
			
			int rem = n;
			while(!queue.isEmpty()) {
				int v = queue.poll();
				rem--;
				for(int nxt : adjlist[v]) {
					for(int j=0;j<26;j++) {
						dp[nxt][j] = Math.max(dp[nxt][j], dp[v][j] + (c[nxt]-'a'==j?1:0));
					}
					deg[nxt]--;
					if(deg[nxt]==0) queue.add(nxt);
				}
			}
			
			if(rem!=0) {
				out.println(-1);
				out.flush();
				return;
			}
			
			int max = 0;
			for(int i=0;i<n;i++) {
				for(int j=0;j<26;j++) {
					max = Math.max(max, dp[i][j]);
				}
			}
			
			out.println(max);
			
		}
		
		out.close();
		
	}
	
	
	

	
	
	
	
	static final Random random=new Random();
	
	static void ruffleSort(long[] a) {
		int n=a.length;//shuffle, then sort 
		for (int i=0; i<n; i++) {
			int oi=random.nextInt(n); long temp=a[oi];
			a[oi]=a[i]; a[i]=temp;
		}
		Arrays.sort(a);
	}
	
	
	
	static class FastScanner{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st = new StringTokenizer("");

		public String next(){
			while(!st.hasMoreElements()){
				try{
					st = new StringTokenizer(br.readLine());
				} catch(IOException e){
					e.printStackTrace();
				}
			}
			return st.nextToken();
		}
		
		public String nextLine() throws IOException {
			return br.readLine();
		}
		
		public int nextInt(){
			return Integer.parseInt(next());
		}

		public int[] readArray(int n){
			int[] a = new int[n];
			for(int i=0;i<n;i++)
				a[i] = nextInt();
			return a;
		}
		
		public long nextLong() {
			return Long.parseLong(next());
		}
		
		public char nextChar() {
			return next().toCharArray()[0];
		}
	}
	
}
