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

from sys import stdin
input= lambda : stdin.readline().strip()
from math import ceil,sqrt,factorial,gcd
from collections import deque
from bisect import bisect_left
def beauty(n,m,s,x,y):
    degree_a=[0 for i in range(n)]
    graph_o={i:[] for i in range(n)}
    for i in range(m):
        x[i]-=1
        y[i]-=1
        graph_o[x[i]].append(y[i])
        degree_a[y[i]]+=1
    arr_a=deque()
    for i in range(n):
        if degree_a[i]==0:
            arr_a.append(i)
    count_11=0
    answer_11=0
    num_1=[[0 for i in range(26)] for i in range(n)]
    while count_11<n and arr_a:
        a_1=arr_a.popleft()
        count_11+=1
        num_1[a_1][ord(s[a_1])-97]+=1
        for i in graph_o[a_1]:
            for j in range(26):
                num_1[i][j]=max(num_1[i][j],num_1[a_1][j])
            degree_a[i]-=1
            if degree_a[i]==0:
                arr_a.append(i)
    if count_11!=n:
        return -1
    else:
        answer_11=0
        for i in range(n):
            answer_11=max(answer_11,max(num_1[i]))
        return answer_11

# print(beauty(1,1,"A",[1],[1])) #-1
# # print(beauty(10,14,"xzyzyzyzqx",[1,2,3,4,2,6,6,2,3,10,4,1,2,3],[2,4,5,5,6,8,5,10,9,9,6,10,8,7]))
# # "BAA", [(0, 1), (2, 0)], 2

n=int(input())
m=int(input())
s=input()
x=[int(i) for i in input().split()]
y=[int(i) for i in input().split()]
print(beauty(n,m,s,x,y))