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
input=lambda : stdin.readline().strip()
from collections import deque
n,m=map(int,input().split())
s=input()
x=[]
y=[]
for i in range(m):
    a,b=map(int,input().split())
    x.append(a)
    y.append(b)
 
def beauty(n,m,s,x,y) :
    l=list(s)
    graph={i:[] for i in range(n)}
    degree=[0 for i in range(n)]
    for i in range(m) :
        x[i]-=1
        y[i]-=1
        graph[x[i]].append(y[i])
        degree[y[i]]+=1
    que=deque()
    for i in range(n) :
        if degree[i]==0 :
            que.append(i)
    
    cnt=0
    ans=0
    dp=[]
    for i in range(n) :
        dp.append([0]*26)
    
    while True :
        if (cnt<n and que) :
            f=que.popleft()
            cnt+=1
            dp[f][ord(l[f])-97]+=1
            for i in graph[f] :
                for j in range(26) :
                    dp[i][j]=max(dp[i][j],dp[f][j])
                degree[i]-=1
                if degree[i]==0 :
                    que.append(i)
        else :
            break
 
    if cnt==n :
        for i in range(n) :
            ans=max(ans,max(dp[i]))
        print(ans)
    else :
        print(-1)
 
beauty(n,m,s,x,y)