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 defaultdict,deque
import sys
input=sys.stdin.readline
n,m=map(int,input().split())
arr=list(input())
q=deque()
res=0
dg=[0 for i in range(n)]
cnt=0
gp=defaultdict(list)
for i in range(m):
    l,k=map(int,input().split())
    l=l-1
    k=k-1
    gp[l].append(k)
    dg[k]=dg[k]+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
lmk=0
lmk=lmk+1
mt=[[0 for i in range(26)] for i in range(n)]
for i in range(n):
	if dg[i]==0:
		q.append(i)
while(cnt <n and q):
    t=q.popleft()
    cnt=cnt+1
    mt[t][ord(arr[t])-97]+=1
    for i in gp[t]:
        for j in range(26):
            mt[i][j]=max(mt[i][j],mt[t][j])
        dg[i]-=1
        if dg[i]==0:
            q.append(i)
if(cnt!=n):
    print(-1)
else:
    for i in range(0,n):
        res=max(res,max(mt[i]))
    print(res)