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.*;
import java.math.*;
 
public class Main
{
    static int bfs(char arr[], int n, int deg[], ArrayList<Integer> l[]){
        Queue<Integer> q  = new LinkedList<>();
        int res = 0, dp[][] = new int[n + 1][26];

        for(int i = 1; i <= n; i++)
            if(deg[i] == 0){
                int idx = (int)(arr[i] - 'a');
                dp[i][idx]++;
                q.add(i);
            }

        while(!q.isEmpty()){
            int node = q.poll();

            for(int adj : l[node]){
                if(deg[adj] == 0)
                    return -1;

                int idx = (int)(arr[adj] - 'a');

                for(int j = 0; j < 26; j++){
                    if(j == idx)
                        dp[adj][idx] = Math.max(dp[adj][idx], 1 + dp[node][idx]);
                    else
                        dp[adj][j] = Math.max(dp[adj][j], dp[node][j]);
                }

                deg[adj]--;
                if(deg[adj] == 0)
                    q.add(adj);

                //trace(node, deg);
            }
        }

        for(int i = 1; i <= n; i++)
            if(deg[i] != 0)
                return -1;

        for(int i = 1; i <= n; i++)
            for(int j = 0; j < 26; j++)
                res = Math.max(res, dp[i][j]);

        return res;
    }

    public static void process(int test_number)throws IOException
    {
        int n = ni(), m = ni(), deg[] = new int[n + 1], res = 0;
        char arr[] = (" "+nln()).toCharArray();

        ArrayList<Integer> l[] = new ArrayList[n + 1];
        for(int i = 1; i <= n; i++)
            l[i] = new ArrayList<>();

        for(int i = 1; i <= m; i++){
            int u = ni(), v = ni();
            l[u].add(v);

            deg[v]++;
        }

        res = bfs(arr, n, deg, l);

        pn(res);
    }

    static final long mod = (long)1e9+7l;
    
    static FastReader sc;
    static PrintWriter out;
    public static void main(String[]args)throws IOException
    {
        out = new PrintWriter(System.out);
        sc = new FastReader();
 
        long s = System.currentTimeMillis();
        int t = 1;
        //t = ni();
        for(int i = 1; i <= t; i++)
            process(i);
 
        out.flush();
        System.err.println(System.currentTimeMillis()-s+"ms");
    }

    static void trace(Object... o){ System.err.println(Arrays.deepToString(o)); };
    static void pn(Object o){ out.println(o); }
    static void p(Object o){ out.print(o); }
    static int ni()throws IOException{ return Integer.parseInt(sc.next()); }
    static long nl()throws IOException{ return Long.parseLong(sc.next()); }
    static double nd()throws IOException{ return Double.parseDouble(sc.next()); }
    static String nln()throws IOException{ return sc.nextLine(); }
    static long gcd(long a, long b)throws IOException{ return (b==0)?a:gcd(b,a%b);}
    static int gcd(int a, int b)throws IOException{ return (b==0)?a:gcd(b,a%b); }
    static int bit(long n)throws IOException{ return (n==0)?0:(1+bit(n&(n-1))); }
    
    static class FastReader{ 
        BufferedReader br; 
        StringTokenizer st; 
  
        public FastReader(){ 
            br = new BufferedReader(new InputStreamReader(System.in)); 
        } 
  
        String next(){ 
            while (st == null || !st.hasMoreElements()){ 
                try{ st = new StringTokenizer(br.readLine()); } catch (IOException  e){ e.printStackTrace(); } 
            } 
            return st.nextToken(); 
        } 
  
        String nextLine(){ 
            String str = ""; 
            try{ str = br.readLine(); } catch (IOException e) { e.printStackTrace(); } 
            return str; 
        } 
    } 
}
