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 functools import lru_cache
from collections import defaultdict, deque 
input = sys.stdin.readline


def get_adj_list(edges, n):
    
    adj_list = defaultdict(list)
    incoming = [0] * n
    
    for src, dst in edges:
        adj_list[src].append(dst)
        incoming[dst] += 1
        
    return adj_list, incoming

def letter_to_int(letter):
    return ord(letter) - 97
    
def solve(letters, edges, n):
    
    adj_list, incoming = get_adj_list(edges, n)
    
    nodes = []
    for node in range(n):
        if incoming[node] == 0:
            nodes.append(node)
    
    longest_paths = [[0] * 26 for _ in range(n)]
    for i, letter in enumerate(letters):
        longest_paths[i][letter_to_int(letter)] = 1

    num_visited_nodes = 0
    max_path = 1
    while nodes:
        node = nodes.pop()
        num_visited_nodes += 1
        
        for child in adj_list[node]:
            incoming[child] -= 1
            for i in range(26):
                delta = 1 if i == letter_to_int(letters[child]) else 0
                longest_paths[child][i] = max(longest_paths[child][i], longest_paths[node][i] + delta)
            max_path = max(max_path, max(longest_paths[child]))
            
            if incoming[child] == 0:
                nodes.append(child)
        
    return max_path if n == num_visited_nodes else -1
    

if __name__ == "__main__":
    n, m = [int(val) for val in input().split()]
    
    letters = [char for char in input().split()][0]
    edges = []
    for _ in range(m):
        edge = [int(val)-1 for val in input().split()]
        edges.append(edge)
        
    print(solve(letters, edges, n))
        
                
