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.math.*;
import java.util.*;

public class codeforces {
    static int MAXN = (int) 1e5;
    static int max = 9999;
    static int n;
    static int m;
    public static int[] sieve;
    public static int[] primes;
    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() throws Exception {
            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;
        }

        int[] readArrayInt(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = Integer.parseInt(next());
            }
            return a;
        }

        long[] readArrayLong(long n) {
            long[] a = new long[(int) n];
            for (int i = 0; i < n; i++) {
                a[i] = Long.parseLong(next());
            }
            return a;
        }

        static void debug(Object... args) {
            System.out.println(Arrays.deepToString(args));
        }

        void swapInt(int a, int b) {
            int temp = a;
            a = b;
            b = temp;
        }

        void swapLong(long a, long b) {
            long temp = a;
            a = b;
            b = temp;
        }

        public void setSieve() {
            primes = new int[MAXN];
            sieve = new int[MAXN];
            int i, j, k = 0;
            for (i = 2; i < MAXN; ++i)
                if (sieve[i] == 0) {
                    primes[k++] = i;
                    for (j = i; j < MAXN; j += i) {
                        sieve[j] = i;
                    }
                }
        }

        int gcd(int a, int b) {
            if (b == 0) {
                return a;
            }
            return gcd(b, a % b);
        }

        long gcd(long a, long b) {
            if (b == 0) {
                return a;
            }
            return gcd(b, a % b);
        }
    }
    static int count = 0;
    public static void main(String[] args) throws Exception {
        FastReader scan = new FastReader();
        PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
        int n = scan.nextInt();
        int m = scan.nextInt();
        String s = scan.nextLine();
        int[] x = new int[m];
        int[] y = new int[m];
        for(int i = 0; i < m; i++) {
        	x[i] = scan.nextInt();
        	y[i] = scan.nextInt();
        }
        out.println(solve(n,m,x,y,s));
        out.flush();
	}
    public static List<Integer> adj[];
    public static int dp[][] = new int[300001][26];
	public static int kk[];
    public static int solve(int n,int m,int[] x,int[] y,String s) {
    	String k = s;
    	adj = new LinkedList[n];
		kk = new int[n];
    	int degree[] = new int[n];
		for(int i=0; i<n; i++) {
			kk[i] = k.charAt(i)-'a';
			adj[i] = new LinkedList<>();
		}
		for(int i=0; i<m; i++) {
			int a = x[i]-1;
			int b = y[i]-1;
			adj[a].add(b);
			degree[b]++;
		}
		ArrayDeque<Integer> q = new ArrayDeque<>();
		for(int i=0; i<n; i++) {
			if(degree[i]!=0) continue;
			q.add(i);
		}
		int check =0;
		while(!q.isEmpty()) {
			check++;
			int curr = q.poll();
			dp[curr][kk[curr]]++;
			for(int i:adj[curr]) {
				for(int j=0; j<26; j++) {
					dp[i][j] = Math.max(dp[i][j],dp[curr][j]);
				}
				degree[i]--;
				if(degree[i]==0)q.add(i);
			}
		}
		
		if(check<n) return -1;
		else {
			int max = 0;
			for(int i=0;i <n; i++) {
				for(int j=0; j<26; j++) {
					max=Math.max(max, dp[i][j]);
				}
			}
			return max;
		}
    }
}