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.



from sys import stdin, stdout 

from collections import deque

def beauty(n,m,s,x,y):

    l=list(s)
    D=[0 for _ in range(n)]
    G={_:[] for _ in range(n)}
    for _ in range(m):
        a,b=x[_],y[_]
        a-=1
        b-=1
        G[a].append(b)
        D[b]+=1
    q=deque()
    for _ in range(n):
        if D[_]==0:
            q.append(_)

    c=0
    ans=0
    DP=[[0 for _ in range(26)] for _ in range(n)]
    while c<n and q:
        x=q.popleft()
        c+=1
        DP[x][ord(l[x])-97]+=1
        for _ in G[x]:
            for __ in range(26):
                DP[_][__]=max(DP[_][__],DP[x][__])
            D[_]-=1
            if D[_]==0:
                q.append(_)

    if c!=n:
        print(-1)
    else:
        for _ in range(n):
            ans=max(ans,max(DP[_]))
        print(ans)
        
        
        
n,m=[int(_) for _ in stdin.readline().split()]
s=input()
x,y=[],[]
for _ in range(m):
	a,b=map(int,stdin.readline().strip().split())
	x.append(a)
	y.append(b)
beauty(n,m,s,x,y)