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
"""
Дан граф из n вершин и m ориентированных ребер. В каждой вершине записана некоторая строчная
латинская буква. Определим величину пути как наибольшее количество раз, которое какая-то буква
встречалась на этом пути. Например, если буквы на пути образуют строку «abaca», то величина
этого пути равна 3. Ваша задача — найти путь с наибольшей величиной.
"""
from collections import deque
import copy

class Node:
    def __init__(self):
        self.adjacent_out = list()
        self.indegree = 0
        self.lengths = dict()

        for i in 'qwertyuiopasdfghjklzxcvbnm':
            self.lengths[i] = 0

class Graph:
    def __init__(self, N, string):
        self.vertices = dict()
        self.string = ' ' + string
        self.roots = set()
        self.max = 0
        self.N = N

        for i in range(1, N + 1):
            self.vertices[i] = Node()
    
    def add_edge(self, a, b):
        self.vertices[a].adjacent_out += [b]
        self.vertices[b].indegree += 1
        if b in self.roots:
            self.roots.remove(b)
        if not self.vertices[a].indegree:
            self.roots.add(a)
    
    def topo_sort(self, queue=deque()):
        for i in self.roots: # база
            self.vertices[i].lengths[self.string[i]] += 1
        proceeded = 0
        while queue:
            q = queue.popleft()
            proceeded += 1
            for neighbour in self.vertices[q].adjacent_out:
                self.vertices[neighbour].indegree -= 1
                self.vertices[neighbour].lengths = copy.deepcopy(self.vertices[q].lengths)
                self.vertices[neighbour].lengths[self.string[neighbour]] += 1
                if self.max < self.vertices[neighbour].lengths[self.string[neighbour]]:
                    self.max = self.vertices[neighbour].lengths[self.string[neighbour]]
                if not self.vertices[neighbour].indegree:
                    queue.append(neighbour)
        if proceeded != self.N:
            self.max = -1
    
    def get_max(self):
        self.topo_sort(deque(self.roots))
        return self.max

N, M = map(int, input().split())
string = input()
graph = Graph(N, string)
for _ in range(M):
    graph.add_edge(*list(map(int, input().split())))
print(graph.get_max())