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 collections import deque

graph = {}
ind = []
dp = []

def topological_sort(n, s):
    global graph
    global ind
    global dp
    visited = 0
    queue = deque()
    ans = 0

    for i in range(1, n + 1):
        if ind[i] == 0:
            queue.append(i)
            dp[i][ord(s[i - 1]) - 97] = 1

    while queue:
        v = queue.popleft()
        visited += 1
        ans = max(ans, dp[v][ord(s[v - 1]) - 97])
        
        for u in graph[v]:
            ind[u] -= 1
            if ind[u] == 0:
                queue.append(u)

            curr = ord(s[u - 1]) - 97
            for i in range(26):
                if i == curr:
                    dp[u][i] = max(dp[u][i], 1 + dp[v][i])
                else:
                    dp[u][i] = max(dp[u][i], dp[v][i])
                    
                ans = max(ans, dp[u][i])
    
    if visited != n:
        return -1
    return ans

if __name__ == '__main__':
    n, m = list(map(int, input().rstrip().split()))
    s = input()

    graph = [[] for i in range(n + 1)]
    ind = [0 for i in range(n + 1)]
    dp = [[0 for i in range(26)] for j in range(n + 1)]

    for i in range(m):
        so, de = list(map(int, input().rstrip().split()))
        graph[so].append(de)
        ind[de] += 1
    print(topological_sort(n, s))