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
#   J A I  S H R E E  R A M   #


import math, sys, collections, functools, time, itertools;

# sys.setrecursionlimit(10**6)


def Read_Ints()     : return map(int, input().strip().split())
def Read_Array()    : return list(Read_Ints())
def Read_Strings()  : return list(input().strip().split())
def printxsp(*args) : return print(*args, end="")
def printsp(*args)  : return print(*args, end=" ")



d4i = [-1, +0, +1, +0]; d8i = [-1, -1, +0, +1, +1, +1, +0, -1]; 
d4j = [+0, +1, +0, -1]; d8j = [+0, +1, +1, +1, +0, -1, -1, -1];


# >>>>>>--->>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>





def solveEachTest(_TestCase):
    # printsp("Case #{}: ".format(_TestCase)) 
    n, m = Read_Ints()
    adj = collections.defaultdict(list)
    indeg = [0 for x in range(n+1)]
    s = input();

    for i in range(m):
        a, b = Read_Ints()
        adj[a].append(b);
        indeg[b] += 1;

    que = collections.deque();

    ans = 0; 
    dp = [[0 for x in range(26)] for xx in range(n+1)]
    indegCopy = indeg.copy()
    for j in range(26):
        ch = chr(ord('a') + j)
        indeg = indegCopy.copy()
        for i in range(1, n+1):
            if indeg[i] == 0:
                que.append(i)
                dp[i][j] = s[i-1] == ch

        cnt = 0
        while (len(que)):
            node = que.popleft()
            for i in adj[node]:
                dp[i][j] = dp[node][j] + (s[i-1] == ch);
                ans = max(ans, dp[i][j])
                indeg[i] -= 1
                if indeg[i] == 0:
                    que.append(i)
            cnt += 1
            if cnt > n:
                break
        if cnt > n:
            break
    # print(cnt)
    print(ans if cnt == n else -1)
    






_T0T4 = 1;
# _T0T4 = int(input()) 
for _TestCase in range(1, _T0T4 + 1): 
    solveEachTest(_TestCase)


# Udit "luctivud" Gupta 
# linkedin :  https://www.linkedin.com/in/udit-gupta-1b7863135/

