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.*;
import java.math.*;
import java.lang.*;
 
import static java.lang.Math.*;
 
public class Main implements Runnable
{
    static class InputReader
    {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;
        private BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

        public InputReader(InputStream stream)
        {
            this.stream = stream;
        }
        
        public int read()
        {
            if (numChars==-1) 
                throw new InputMismatchException();
            
            if (curChar >= numChars)
            {
                curChar = 0;
                try 
                {
                    numChars = stream.read(buf);
                }
                catch (IOException e)
                {
                    throw new InputMismatchException();
                }
                
                if(numChars <= 0)               
                    return -1;
            }
            return buf[curChar++];
        }
     
        public String nextLine()
        {
            String str = "";
            try
            {
                str = br.readLine();
            }
            catch (IOException e)
            {
                e.printStackTrace();
            }
            return str;
        }
        public int nextInt()
        {
            int c = read();
            
            while(isSpaceChar(c)) 
                c = read();
            
            int sgn = 1;
            
            if (c == '-') 
            {
                sgn = -1;
                c = read();
            }
            
            int res = 0;
            do 
            {
                if(c<'0'||c>'9') 
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c)); 
            
            return res * sgn;
        }
        
        public long nextLong() 
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') 
            {
                sgn = -1;
                c = read();
            }
            long res = 0;
            
            do 
            {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            while (!isSpaceChar(c));
                return res * sgn;
        }
        
        public double nextDouble() 
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') 
            {
                sgn = -1;
                c = read();
            }
            double res = 0;
            while (!isSpaceChar(c) && c != '.') 
            {
                if (c == 'e' || c == 'E')
                    return res * Math.pow(10, nextInt());
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            }
            if (c == '.') 
            {
                c = read();
                double m = 1;
                while (!isSpaceChar(c)) 
                {
                    if (c == 'e' || c == 'E')
                        return res * Math.pow(10, nextInt());
                    if (c < '0' || c > '9')
                        throw new InputMismatchException();
                    m /= 10;
                    res += (c - '0') * m;
                    c = read();
                }
            }
            return res * sgn;
        }
        
        public String readString() 
        {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            StringBuilder res = new StringBuilder();
            do 
            {
                res.appendCodePoint(c);
                c = read();
            } 
            while (!isSpaceChar(c));
            
            return res.toString();
        }
     
        public boolean isSpaceChar(int c) 
        {
            if (filter != null)
                return filter.isSpaceChar(c);
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }
     
        public String next() 
        {
            return readString();
        }
        
        public interface SpaceCharFilter 
        {
            public boolean isSpaceChar(int ch);
        }
    }
    
    public static void main(String args[]) throws Exception
    {
        new Thread(null, new Main(),"Main",1<<26).start();
    }
    void dfsAns(int i) {
        vis[i] = 2;
        for(int j : adj[i]) {
            if(vis[j] == 0)
                dfsAns(j);
            else if(vis[j] == 2) {
                flag = 1;
                return;
            }
            if(flag == 1)
                return;
            for(int k = 0; k < 26; ++k) {
                freq[i][k] = max(freq[i][k], freq[j][k]);
            }
        }
        freq[i][s[i] - 'a']++;
        int maxFreq = 0;
        for(int j = 0; j < 26; ++j) {
            if(freq[i][j] > maxFreq)
                maxFreq = freq[i][j];
        }
        ans = max(ans, maxFreq);
        vis[i] = 1;
    }
    void dfs(int i) {
        vis[i] = 1;
        for(int j : adj[i]) {
            if(vis[j] == 0)
                dfs(j);
        }
        topoSort.add(i);
    }
    int flag = 0;
    ArrayList<Integer> adj[];
    ArrayList<Integer> topoSort = new ArrayList<>();
    int vis[];
    char s[];
    int freq[][];
    int ans = 0;
    public void run()
    {
        InputReader sc= new InputReader(System.in);
        PrintWriter w= new PrintWriter(System.out);
        
        int n = sc.nextInt();
        int m = sc.nextInt();

        s = sc.next().toCharArray();

        adj = new ArrayList[n];

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

        for(int i = 0; i < m; ++i) {
            int u = sc.nextInt() - 1;
            int v = sc.nextInt() - 1;
            adj[u].add(v);
        }

        vis = new int[n];

        for(int i = 0; i < n; ++i) {
            if(vis[i] == 0) {
                dfs(i);
            }
        }

        vis = new int[n];
        freq = new int[n][26];

        for(int i = n - 1; i >= 0; --i) {
            int cur = topoSort.get(i);
            if(vis[cur] == 0) {
                dfsAns(cur);
            }
        }

        if(flag == 1)
            w.print("-1");
        else
            w.print(ans);

        w.close();      
    }
}