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.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Codechef
{
    long MOD = 1000000007L;
    int INF = 1000000001;
    public class Obj implements Comparable<Obj>{
        public int t;
        public int h;
        
        public Obj(int t_, int h_){
            t = t_;
            h = h_;
        }
        
        public int compareTo(Obj other){
            return t - other.t;
        }
        
    }
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
        Codechef cf = new Codechef();
        cf.Solve();
	}
	
    char[] arr;
    ArrayList<ArrayList<Integer>> con;
    boolean[] vis;
    boolean[] st;
    int[][] dp;
	
	public void Solve(){
        MyScanner sc = new MyScanner();
        int n = sc.nextInt();
        int m = sc.nextInt();
        String s = sc.nextLine();
        arr = s.toCharArray();
        con = new ArrayList<ArrayList<Integer>>();
        for (int i = 0; i < n; ++i){
            con.add(new ArrayList<Integer>());
        }
        for (int i = 0; i < m; ++i){
            int u = sc.nextInt();
            int v = sc.nextInt();
            u--;
            v--;
            con.get(u).add(v);
        }
        vis = new boolean[n];
        st = new boolean[n];
        for (int i = 0; i < n; ++i){
            if (!vis[i]){
                boolean cycle = isCycle(i);
                if (cycle){
                    System.out.println(-1);
                    return;
                }
            }
        }
        
        dp = new int[n][26];
        vis = new boolean[n];
        for (int i = 0; i < n; ++i){
            if (!vis[i]){
                dfs(i);
            }
        }
        int best = 0;
        for (int i = 0; i < n; ++i){
            // System.out.println(i);
            for (int j = 0; j < 26; ++j){
                // System.out.print((char)(j + 'a') + " : " + dp[i][j] + " -- ");
                best = Math.max(best, dp[i][j]);
            }
            // System.out.println();
        }
        System.out.println(best);
	}
	
	public boolean isCycle(int cur){
	    vis[cur] = true;
	    st[cur] = true;
	    ArrayList<Integer> co = con.get(cur);
	    boolean isCycle = false;
	    for (int u : co){
	        if (!vis[u]){
	            isCycle |= isCycle(u);
	            if (isCycle){
	                return true;
	            }
	        }else{
	            if (st[u]){
	                return true;
	            }
	        }
	    }
	    st[cur] = false;
	    return isCycle;
	}
	
	public void dfs(int cur){
	    vis[cur] = true;
	    ArrayList<Integer> co = con.get(cur);
	    int idx = (int)(arr[cur] - 'a');
	    dp[cur][idx] = Math.max(dp[cur][idx], 1);
	    for (int u : co){
	        int[] child;
	        if (vis[u]){
	            child = dp[u];
	        }else{
	            dfs(u);
	            child = dp[u];
	        }
	        for (int i = 0; i < 26; ++i){
	            if (i != idx){
	                dp[cur][i] = Math.max(dp[cur][i], dp[u][i]);
	            }else{
	                dp[cur][i] = Math.max(dp[cur][i], 1 + dp[u][i]);
	            }
	        }
	    }
	}
	

	//-----------PrintWriter for faster output---------------------------------
   public static PrintWriter out;
      
   //-----------MyScanner class for faster input----------
   public static class MyScanner {
      BufferedReader br;
      StringTokenizer st;
 
      public MyScanner() {
         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();
      }
 
      int nextInt() {
          return Integer.parseInt(next());
      }
 
      long nextLong() {
          return Long.parseLong(next());
      }
 
      double nextDouble() {
          return Double.parseDouble(next());
      }
 
      String nextLine(){
          String str = "";
      try {
         str = br.readLine();
      } catch (IOException e) {
         e.printStackTrace();
      }
      return str;
      }

   }
}