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 zero_func(nums):
    l=[]
    for i in range(nums):
        k=[]
        for j in range(0,26):
            k.append(0)
        l.append(k)
    return l
def degue_func(nums1,check):
    q = deque()
    for i in range(nums1):
        if check[i] == 0:
            q.append(i)
    return q
 
 
def beauty(n, m, s, x, y):
    Degree=[0]*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])
        Degree[y[i]]+=1
    check=Degree
    
    check_ans=degue_func(n,check)
    ans=zero_func(n)
 
    while c<n and check_ans:
        a=check_ans.popleft()
        c+=1
        ans[a][ord(s[a])-97]+=1
        for i in graph[a]:
            for j in range(0,26):
                ans[i][j]=max(ans[i][j],ans[a][j])
            Degree[i]-=1
            if Degree[i]==0:
                check_ans.append(i)
    if c!=n:
        return -1
    else:
        result=0
        for i in range(n):
            result=max(result,max(ans[i]))
        return result
 
n1,m1=map(int,input().split())
s1=input()
listX=[]
listY=[]
for t in range(m1):
    x1,y1=map(int,input().split())
    listX.append(x1)
    listY.append(y1)
x=beauty(n1,m1,s1,listX,listY)
print(x)