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.
The first line contains two positive integers $n, m$ ($1 \leq n, m \leq 300\,000$), 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 $x, y$ ($1 \leq x, y \leq n$), 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.
In the first sample, the path with largest value is $1 \to 3 \to 4 \to 5$. The value is $3$ because the letter 'a' appears $3$ times.
5 4
abaca
1 2
1 3
3 4
4 5
3
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4
-1
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
4
1 1
f
1 1
-1
10 50
ebibwbjihv
1 10
1 2
5 4
1 8
9 7
5 6
1 8
8 7
2 6
5 4
1 9
3 2
8 3
5 6
5 9
2 4
2 7
3 9
1 2
1 7
1 10
3 7
1 8
3 10
8 6
1 7
10 6
1 6
5 8
1 5
2 10
3 9
5 8
8 3
3 7
5 2
1 10
1 4
5 3
3 2
1 2
5 8
10 4
2 10
8 2
1 9
1 8
1 2
3 4
1 8
2
13 37
dwpzcppjmhkmz
2 6
3 6
6 7
6 7
6 7
6 7
6 8
6 8
6 8
6 8
4 6
4 6
5 6
4 6
4 6
6 9
6 9
6 10
6 10
6 10
6 10
4 6
1 6
1 6
10 11
6 11
1 6
6 12
6 12
6 12
6 13
6 13
6 13
6 13
3 6
2 6
2 6
3
5 8
tetqw
2 1
4 4
5 5
5 2
4 5
1 5
1 5
1 1
-1
5 8
reeet
4 3
2 5
4 2
2 4
4 2
5 2
3 3
3 4
-1
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.util.StringTokenizer;
 
import java.util.HashMap;
import java.util.HashSet;
 
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;
    }
}
public class Main{
    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());
    }
}