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
from sys import stdin
import functools
input=lambda:stdin.readline().strip()
def beautifulPath(n,m,s,x,y):
    degree=[0]*n
    graph=defaultdict(list)
    for i in range(m):
        a,b=x[i],y[i]
        a-=1
        b-=1
        graph[a].append(b)
        degree[b]+=1
    q=deque()

    for i in range(n):
        # If no incoming edges then append to queue
        if degree[i]==0:
            q.append(i)

    count=0
    dp=[[0]*26 for i in range(n)]
    while count<n and q:
        a=q.popleft()
        count+=1
        dp[a][ord(s[a])-97]+=1
        for i in graph[a]:
            for j in range(26):
                dp[i][j]=max(dp[a][j],dp[i][j])
            degree[i]-=1
            if degree[i]==0:
                q.append(i)
    if count!=n:
        return -1
    else:
        # return max of dp
        return functools.reduce(lambda x,y: x if x>y else y,set(max(dp[i]) for i in range(n)))


n,m=map(int,input().split())
s=input()
xList=[]
yList=[]
for i in range(m):
    x,y=map(int,input().split())
    xList.append(x)
    yList.append(y)
finalPath=beautifulPath(n,m,s,xList,yList)
print(finalPath)
