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

input= lambda: sys.stdin.buffer.readline().decode('utf-8').rstrip('\r\n')
 
from array import array

inp= lambda: int(input())
mi = lambda x=int : map(x, input().split())
arr= lambda d='i',x=int: array(d,mi(x))
li = lambda x=int: list(mi(x))
lb = lambda: list(map(int, input()))
ls = lambda: list(input())
bi = lambda n: bin(n).replace("0b", "")

yn = ['No', 'Yes']
YN = ['NO', 'YES']
YY = "YES"
NN = "NO"
yy = "Yes"
nn = "No"
inf= sys.maxsize
mod=998244353
alp='abcdefghijklmnopqrstuvwxyz'

from itertools import accumulate as cml,permutations,combinations
from collections import Counter as ctr, deque as dq,defaultdict as dd
from bisect import bisect_left as bl,bisect_right as br
from math import ceil, gcd, log, sqrt
import heapq as hq

mod=10**9+7


mod=1000000007

def main(kase):
    n,m=mi()
    s=input()
    grp=[[] for i in range(n)]

    for i in range(m):
        x,y=mi(lambda x: int(x)-1)
        grp[x].append(y)
    
    clr=[-1]*n
    cc=1
    dp=[[0 for i in range(26)] for j in range(n)]
    flag=False
    for i in range(n):
        if clr[i]==-1:
            q=[i]
            while q:
                nd=q[-1]
                if clr[nd]==-1:
                    for x in grp[nd]:
                        if clr[x]==0:
                            flag=True
                            break
                        elif clr[x]==-1:
                            q.append(x)
                    clr[nd]=0
                elif clr[nd]==0:
                    for x in grp[nd]:
                        for j in range(26):
                            dp[nd][j]=max(dp[nd][j],dp[x][j])
                    
                    dp[nd][(ord(s[nd])-ord('a'))]+=1
                    clr[nd]=cc
                    q.pop()
                if flag:
                    break
            cc+=1
            if flag:
                break
    if flag:
        print(-1)
    else:
        print(max(max(x) for  x in dp))
        # for x in dp:
        #     print(x)
    



    pass
if __name__ == "__main__":
    test_Cases=1
    # test_Cases=inp()
    for i in range(test_Cases):
        main(i)