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.BufferedWriter;
import java.io.FileInputStream;
import java.io.FileWriter;
import java.io.FileNotFoundException;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.StringTokenizer;

import java.util.HashMap;
import java.util.HashSet;

public class Main{
    //static IOHandler System=new IOHandler();
    public static void main(String[] args) throws IOException{
        BufferedReader input=new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st;

        int n, m;
        st=new StringTokenizer(input.readLine().trim());
        n=Integer.parseInt(st.nextToken());
        m=Integer.parseInt(st.nextToken());
        String s=input.readLine().trim();
        Graph graph=new Graph(n, s);
        while(m-->0){
            int x, y;
            st=new StringTokenizer(input.readLine().trim());
            x=Integer.parseInt(st.nextToken());
            y=Integer.parseInt(st.nextToken());
            graph.addEdge(x, y);
        }
        System.out.println(graph.maximumCost());
    }
}
class Graph{
    int n;
    String s;
    HashMap<Integer, HashSet<Integer>> adjacencyMatrix;
    int[][] costArray;
    Graph(int n, String s){
        this.n=n;
        this.s=s;
        adjacencyMatrix=new HashMap<Integer, HashSet<Integer>>();
        costArray=new int[n][26];
    }
    void addEdge(int x, int y){
        x-=1;
        y-=1;
        if(!adjacencyMatrix.containsKey(x))
            adjacencyMatrix.put(x, new HashSet<Integer>());
        adjacencyMatrix.get(x).add(y);
    }
    int maximumCost(){
        boolean[] isVisitedWhole=new boolean[n];
        boolean[] isVisitedInPath=new boolean[n];
        for(int i=0;i<n;i++)
            if(!isVisitedWhole[i] && hasCycle(i, isVisitedWhole, isVisitedInPath))
                return -1;
        
        boolean[] isVisited=new boolean[n];
        for(int i=0;i<n;i++)
            if(!isVisited[i])
                dfs(i, isVisited);
        int costMax=-1;
        for(int i=0;i<n;i++)
            for(int j=0;j<26;j++)
                costMax=Math.max(costMax, costArray[i][j]);
        return costMax;
    }
    void dfs(int x, boolean[] isVisited){
        isVisited[x]=true;

        if(adjacencyMatrix.containsKey(x))
            for(int i: adjacencyMatrix.get(x)){
                    if(!isVisited[i])
                        dfs(i, isVisited);
                    for(int j=0;j<26;j++)
                        costArray[x][j]=Math.max(costArray[x][j], costArray[i][j]);
                }
        costArray[x][(int)(s.charAt(x)-'a')]++;
    }
    boolean hasCycle(int x, boolean[] isVisitedWhole, boolean[] isVisitedInPath){
        if(isVisitedInPath[x])
            return true;
        if(isVisitedWhole[x])
            return false;

        isVisitedWhole[x]=isVisitedInPath[x]=true;
        if(adjacencyMatrix.containsKey(x))
            for(int i: adjacencyMatrix.get(x))
                if(hasCycle(i, isVisitedWhole, isVisitedInPath))
                    return true;
        isVisitedInPath[x]=false;
        return false;
    }
}

class IOHandler{
    FileInputStream in;
    OutputHandler out;
    IOHandler(){
        try{
            in=new FileInputStream("input.txt");
            out=new OutputHandler();
        }
        catch(FileNotFoundException e){}
    }
    class OutputHandler{
        OutputHandler(){
            try{
                BufferedWriter output=new BufferedWriter(new FileWriter("output.txt"));
                output.close();
            }
            catch(IOException e){}
        }
        void print(Object s){
            try{
                BufferedWriter output=new BufferedWriter(new FileWriter("output.txt", true));
                output.write(s.toString());
                output.close();
            }
            catch(IOException e){}
        }
        void println(Object s){
            print(s.toString()+"\n");
        }
        void println(){
            println("");
        }
    }
}
