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 collections import deque
import sys
input=sys.stdin.readline
def beauty(n,m,s,x,y):
    deg=[0 for i in range(n)]
    graph={i:[] for i in range(n)}
    c=0
    for i in range(m):
        x[i]-=1
        y[i]-=1
        graph[x[i]].append(y[i])
        deg[y[i]]+=1
    q=deque()
    for i in range(n):
        if deg[i]==0:
            q.append(i)
    l=[]
    for i in range(n):
        k=[]
        for j in range(26):
            k.append(0)
        l.append(k)
    while(c<n and q):
        a=q.popleft()
        c+=1
        l[a][ord(s[a])-97]+=1
        for i in graph[a]:
            for j in range(26):
                l[i][j]=max(l[i][j],l[a][j])
            deg[i]-=1
            if(deg[i]==0):
                q.append(i)
    if(c!=n):
        return -1
    else:
        result=0
        for i in range(n):
            result=max(result,max(l[i]))
        return result
n,m=map(int,input().split())
s=input()
x=[]
y=[]
for _ in range(m):
    a,b=map(int,input().split())
    x.append(a)
    y.append(b)
print(beauty(n,m,s,x,y))
