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 collections import *

# import itertools
# import math
# import os
# import random
# from bisect import bisect, bisect_left
# from functools import reduce
# from heapq import heapify, heappop, heappush
# from io import BytesIO, IOBase
# from string import *

# region fastio
input = lambda: sys.stdin.readline().rstrip()
sint = lambda: int(input())
mint = lambda: map(int, input().split())
ints = lambda: list(map(int, input().split()))
# print = lambda d: sys.stdout.write(str(d) + "\n")
# endregion fastio

# # region interactive
# def printQry(a, b) -> None:
#     sa = str(a)
#     sb = str(b)
#     print(f"? {sa} {sb}", flush = True)

# def printAns(ans) -> None:
#     s = str(ans)
#     print(f"! {s}", flush = True)
# # endregion interactive

# # region dfsconvert
# from types import GeneratorType
# 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
# # endregion dfsconvert

# MOD = 998244353
# MOD = 10 ** 9 + 7
# DIR = ((-1, 0), (0, 1), (1, 0), (0, -1))

def solve() -> None:
    n, m = mint()
    s = list(map(ord, input()))
    g = [[] for _ in range(n)]
    deg = [0] * n
    for _ in range(m):
        u, v = mint()
        u -= 1
        v -= 1
        g[u].append(v)
        deg[v] += 1
    # print(s)
    cnt = [[0] * 26 for _ in range(n)]
    ans = 0
    q = deque((i for i in range(n) if deg[i] == 0))
    path = 0
    while q:
        path += 1
        
        x = q.popleft()
        cnt[x][s[x] - 97] += 1
        for d in range(26):
            ans = max(ans, cnt[x][d])
        
        for y in g[x]:
            for d in range(26):
                cnt[y][d] = max(cnt[y][d], cnt[x][d])
            deg[y] -= 1
            if deg[y] == 0:
                q.append(y)
    if path != n:
        print(-1)
    else:
        print(ans)

solve()