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.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

public class ProblemD {
  public static void main(String [] args) throws IOException{
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String [] strs = reader.readLine().split(" ");
    int n = Integer.parseInt(strs[0]);
    ArrayList<Integer>[] adjList = new ArrayList[n+1];
    for(int i = 0; i <= n; i++){
      adjList[i] = new ArrayList<Integer>();
    }
    int m = Integer.parseInt(strs[1]);
    char [] nodeChars = reader.readLine().toCharArray();

    for(int i = 0; i < m; i++){
      strs = reader.readLine().split(" ");
      int u = Integer.parseInt(strs[0]);
      int v = Integer.parseInt(strs[1]);
      adjList[u].add(v);
    }
    Map<Integer, int []> dp = new HashMap<>();
    boolean [] cycleDetector = new boolean [n + 1];
    int result = -1;
    for(int i = 1; i <= n; i++){
      int [] freq = solve(i, cycleDetector, dp, nodeChars, adjList);
      if(freq[0] < 0){
        result = -1;
        break;
      }
      for(int j = 0; j < 26; j++){
        result = Math.max(result, freq[j]);
      }
    }
    System.out.println(result);
  }
  private static int [] solve(int u, boolean[] cycleDetector, Map<Integer, int []> dp, char [] nodeChars, ArrayList<Integer>[] adjList){
    if(dp.containsKey(u)){
      return dp.get(u);
    }
    int [] freq = new int[26];
    cycleDetector[u] = true;
    for(int v : adjList[u]){
      if(cycleDetector[v]){
        freq[0] = -2;
        break;
      }
      int [] slaveFreq = solve(v, cycleDetector, dp, nodeChars, adjList);
      if(slaveFreq[0] < 0){
        freq[0] = -2;
        break;
      }
      for(int i = 0; i < 26; i++){
        freq[i] = Math.max(freq[i], slaveFreq[i]);
      }
    }
    freq[nodeChars[u - 1] - 'a']++;
    cycleDetector[u] = false;
    dp.put(u, freq);
    return freq;
  }
}