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 typing import final

n = int(input())
m = int(input())
s = input()
x = [int(item) for item in input().split()]
y = [int(item) for item in input().split()]
graph = dict()
dim1, dim2 = (n+1, 26) 
dp = [[0 for i in range(dim2)] for j in range(dim1)]
visited = [0]*(m*n)
cycle = [0]*(m*n)
ans = 0

def genGraph():
    for item in range(0,m):
        if x[item] in graph:
            graph[x[item]].append(y[item])
        else:
            graph[x[item]] = [y[item]]
    print(graph)
genGraph()

def dfs(node):
    visited[node] = 1
    cycle[node] = 1
    try:
        for i in graph[node]:
            if cycle[i]:
                print("-1")
                exit()
            if not visited[i]:
                dfs(i)
            for j in range(0,26):
                dp[node][j] = max(dp[node][j],dp[i][j])
    except Exception as e: pass
    finally:
        # print(node)
        global ans
        dp[node][ord(s[node-1])-ord('a')] +=1
        for i in range(0,26):
            ans = max(dp[node][i],ans)
        cycle[node]=0
        # for row in dp:
        #     print(row)
        # print(visited)


for node in range(1,n+1):
    if not visited[node]:
        dfs(node)
print(ans)


    