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 defaultdict as dd, deque as q


def beauty(n, m, s, x, y):

   
    vertex,bipartite,z=0,0,0
    node = [0 for i in range(n)]
    grph = dd(list)
    lst=list(s)
    ii=0
    while ii <m:
        grph[x[ii]-1].append(y[ii]-1)
        node[y[ii]-1] += 1
        ii+=1

    que = q()

    for i in range(n):
        if node[i] == 0:
            que.append(i)

    counting ,maxpv=0,0
  

    dp = [[0 for i in range(26)] for i in range(n)]

    while counting < n and que:
        val = que.popleft()
        counting += 1
        dp[val][ord(lst[val])-97] += 1
        for i in grph[val]:
            for j in range(26):
                dp[i][j] = max(dp[i][j], dp[val][j])
            node[i] -= 1
            if node[i] == 0:
                que.append(i)

    if counting != n:
        return -1

    else:
        maxpv = 0
        for i in range(n):
            maxpv = max(maxpv, max(dp[i]))
        return maxpv


n, m = map(int, input().split(" "))
s = input()
list1,list2 = [],[]

for i in range(m):
    num1, num2 = map(int, input().split(" "))
    list1.append(num1)
    list2.append(num2)
def control(a ,b ,c):
    return a==b==c
print(beauty(n, m, s, list1, list2))