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.util.*;
import java.io.*;
public class Solution1{

	public static void topological(int index, char a[], LinkedList<Integer> adj[], LinkedList<Integer> rev[], HashMap<Integer,Integer[]> map, int outdegree[], Queue<Integer> q){
		//System.out.println("\t "+index);
		for(int item:adj[index]){
			for(int i=0;i<26;i++)
				map.get(index)[i] = Math.max( map.get(index)[i],map.get(item)[i] );
		}

		for(int item:rev[index]){
			outdegree[item]-=1;
			if(outdegree[item]==0)
				q.add(item);
		}

		map.get(index)[a[index]-97]+=1;
	}

	public static void main(String args[])throws IOException{

		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

		String input[]=br.readLine().split("\\s");
		int n=Integer.parseInt(input[0]),
			m=Integer.parseInt(input[1]);

		String line=br.readLine();
		char a[]=new char[line.length()+1];
		for(int i=0;i<line.length();i++)
			a[i+1]=line.charAt(i);


		LinkedList<Integer> adj[]=new LinkedList[n+1];
		LinkedList<Integer> rev[]=new LinkedList[n+1];
		HashMap<Integer,Integer[]> map=new HashMap<>();


		for(int i=0;i<=n;i++){
			adj[i]=new LinkedList<>();
			rev[i]=new LinkedList<>();

			Integer arr[]=new Integer[26];
			for(int j=0;j<26;j++)
				arr[j]=0;
			map.put(i, arr);
		}


		for(int i=0;i<m;i++){
			input=br.readLine().split("\\s");
			int u=Integer.parseInt(input[0]),
				v=Integer.parseInt(input[1]);

			adj[u].add(v);
			rev[v].add(u);

		}


		int outdegree[]=new int[n+1];
		for(int i=1;i<=n;i++)
			outdegree[i]=adj[i].size();

		Queue<Integer> q=new LinkedList<>();
		for(int i=1;i<=n;i++)
			if(outdegree[i]==0)
				q.add(i);


		boolean visited[]=new boolean[n+1];
		while(!q.isEmpty()){
			int index=q.poll();
			topological(index, a, adj, rev, map, outdegree, q);
			visited[index]=true;
		}

		for(int i=1;i<=n;i++)
			if(!visited[i]){
				System.out.println(-1);
				return;
			}

		int max=0;
		for(int i=1;i<=n;i++)
			for(int item:map.get(i))
				max=Math.max(max,item);

		System.out.println(max);
	}
}