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.
The first line contains two positive integers $n, m$ ($1 \leq n, m \leq 300\,000$), 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 $x, y$ ($1 \leq x, y \leq n$), 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.
In the first sample, the path with largest value is $1 \to 3 \to 4 \to 5$. The value is $3$ because the letter 'a' appears $3$ times.
5 4
abaca
1 2
1 3
3 4
4 5
3
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4
-1
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
4
1 1
f
1 1
-1
10 50
ebibwbjihv
1 10
1 2
5 4
1 8
9 7
5 6
1 8
8 7
2 6
5 4
1 9
3 2
8 3
5 6
5 9
2 4
2 7
3 9
1 2
1 7
1 10
3 7
1 8
3 10
8 6
1 7
10 6
1 6
5 8
1 5
2 10
3 9
5 8
8 3
3 7
5 2
1 10
1 4
5 3
3 2
1 2
5 8
10 4
2 10
8 2
1 9
1 8
1 2
3 4
1 8
2
13 37
dwpzcppjmhkmz
2 6
3 6
6 7
6 7
6 7
6 7
6 8
6 8
6 8
6 8
4 6
4 6
5 6
4 6
4 6
6 9
6 9
6 10
6 10
6 10
6 10
4 6
1 6
1 6
10 11
6 11
1 6
6 12
6 12
6 12
6 13
6 13
6 13
6 13
3 6
2 6
2 6
3
5 8
tetqw
2 1
4 4
5 5
5 2
4 5
1 5
1 5
1 1
-1
5 8
reeet
4 3
2 5
4 2
2 4
4 2
5 2
3 3
3 4
-1
import sys
from types import GeneratorType
from collections import deque
# input = sys.stdin.readline
# def bootstrap(f, stack=[]):
#     def wrappedfunc(*args, **kwargs):
#         if stack:
#             return f(*args, **kwargs)
#         else:
#             to = f(*args, **kwargs)
#             while True:
#                 if type(to) is GeneratorType:
#                     stack.append(to)
#                     to = next(to)
#                 else:
#                     stack.pop()
#                     if not stack:
#                         break
#                     to = stack[-1].send(to)
#             return to
 
#     return wrappedfunc
# @bootstrap
t=1
for tt in range(t):
    n,m=list(map(int,input().split()))
    s=input()
    a=[[] for nn in range(n+1)]
    a1=a[:]
    c=[0]*(n+1)
    for mm in range(m):
        x,y=list(map(int,input().split()))
        a[x]+=[y]
        a1[y]+=[x]
        c[y]+=1
    q=deque([])
    for i in range(1,n+1):
        if c[i]==0:
            q+=[i]
    cc=0
    aa=[]
    while q:
        x=q.popleft()
        cc+=1
        aa+=[x]
        for i in a[x]:
            c[i]-=1
            if c[i]==0:
                q+=[i]
    if cc<n:
        print(-1)
        continue
    ans=0
    for j in range(26):
        dp=[0]*(n+1)
        for i in aa:
            ans1=0
            if s[i-1]==chr(ord('a')+j):
                ans1+=1
            m=0
            for ii in a1[i]:
                m=max(m,dp[ii])
            ans1+=m
            dp[i]=ans1
        ans=max(ans,max(dp))
    print(ans)