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

def main():
    n, m =  list(map(int, input().split()))
    s = list(map(lambda x:ord(x)-97, input().strip()))
    G = defaultdict(list)
    map_ = [[0]*26 for i in range(n)]
    inc = [0]*n
    for _ in range(m):
        x, y = list(map(int, input().split()))
        G[y].append(x)
        inc[x-1]+=1
    q = deque([i+1 for i in range(n) if inc[i] == 0])
    count = len(q)
    while q:
        a = q.popleft()
        map_[a-1][s[a-1]]+=1
        for nbr in G[a]:
            inc[nbr-1]-=1
            for i in range(26):
                map_[nbr-1][i] = max(map_[a-1][i], map_[nbr-1][i])
            if inc[nbr-1] == 0:
                q.append(nbr)
                count+=1
    if count!= n:
        print(-1)
    else:
        print(max([max(f) for f in map_]))
                

            
        

        
    

            


    




# Set the stack size
threading.stack_size(1 << 27)

# Create and start the main thread
main_thread = threading.Thread(target=main)
main_thread.start()

# Wait for the main thread to complete
main_thread.join()
