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,Counter
import sys
import bisect
import math
input=sys.stdin.readline
mod=1000000007
def beauty(n,m,s,x,y):
    b=0
    l=s
    l1=x
    l2=y
    f1=[]
    h1=defaultdict(list)
    for i in range(n):
        f1.append(0)
    
    for i in range(m):
        a,b=l1[i],l2[i]
        a-=1
        b-=1
        h1[a].append(b)
        f1[b]+=1

    q=deque()
    for i in range(n):
      if f1[i]==0:
        q.append(i)
    count=0
    ans=0

    arr1=[[0 for i in range(26)] for i in range(n)]
    while count<n and q:
      x=q.popleft()
      count+=1
      arr1[x][ord(l[x])-97]+=1
      for i in h1[x]:
        for j in range(26):
          arr1[i][j]=max(arr1[i][j],arr1[x][j])
        f1[i]-=1
        if f1[i]==0:
          q.append(i)
    for i in range(1):
        if(b==0):
            b=l1[0]
        else:
            b=l1[-1]
    if count!=n:
      print(-1)
    else:
      ans=0
      for i in range(n):
        ans=max(ans,max(arr1[i]))
      print(ans)
n,m=map(int,input().split())
s=input()
l1=[]
l2=[]
for i in range(m):
    a,b=map(int,input().split())
    l1.append(a)
    l2.append(b)
beauty(n,m,s,l1,l2)
