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 math import *
from collections import *
import math, os, sys, heapq, bisect, random
from functools import lru_cache
from itertools import *
def inp(): return sys.stdin.readline().rstrip("\r\n")
def out(var): sys.stdout.write(str(var))  # for fast output, always take string
def inpu(): return int(inp())
def lis(): return list(map(int, inp().split()))
def stringlis(): return list(map(str, inp().split()))
def sep(): return map(int, inp().split())
def strsep(): return map(str, inp().split())
def fsep(): return map(float, inp().split())
M,M1=1000000007,998244353

def main():
    how_much_noob_I_am = 1
    #how_much_noob_I_am = inpu()
    for case in range(how_much_noob_I_am):
        n,m = sep()
        s= inp()
        d=defaultdict(list)
        for i in range(m):
            x,y=sep()
            d[x].append(y)
            #d[y].append(x)
        q=deque()
        visited = [0]*(n+1)
        ans = 0
        def dfs(visited, d, curr, arr, s):
            nonlocal ans
            #print(arr)
            ans = max(ans, max(arr))
            visited[curr] = 1
            for j in d[curr]:
                if visited[j] == 0:
                    list1 = arr.copy()
                    list1[ord(s[j - 1]) - 97] += 1
                    dfs(visited, d, j, list1, s)
                else:
                    #print(j,curr)
                    if visited[j] == 1:
                        return True
            visited[curr] = 2
            return False
        for i in range(1,n+1):
            if visited[i]==0:
                arr=[0]*(26)
                arr[ord(s[i - 1]) - 97] += 1
                if dfs(visited,d,i,arr,s):
                    print(-1)
                    break

        else:
            print(ans)
            """ans=0
            visited=[False]*(n+1)
            for i in range(1,n+1):
                if not visited[i]:
                    visited[i] = True
                    arr=[0]*(26)
                    arr[ord(s[i - 1]) - 97] += 1
                    q.append([i,arr])
                    while(q):
                        s,list1=q.popleft()
                        ans = max(ans,max(arr))
                        for j in d[s]:
                            if not visited[j]:
                                """
if __name__ == '__main__':
    main()
