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,os,io
from sys import stdin,stdout
from math import log, gcd, ceil
from collections import defaultdict, deque, Counter
from heapq import heappush, heappop
from bisect import bisect_left , bisect_right
import math 
 
if(os.path.exists('input.txt')):
    sys.stdin = open("input.txt","r") ; sys.stdout = open("output.txt","w") 
# else:
#     input = io.BytesIO(os.read(0, os.fstat(0).st_size)).readline
 
def beautifulpath(n,adj,s):
    queue = deque()
    indeg = [0]*(n)
    for i in range(n):
        for j in adj[i]:
            indeg[j]+=1
    
    # print(indeg)
    
    for i in range(n):
        if(indeg[i]==0):
            queue.append(i)
            
    # print(queue)
 
    topo = []
    count = 0
    dp = [[0 for i in range(26)] for i in range(n)]
    
    while queue:
        start = queue.popleft()
        topo.append(start)
        count+=1
        dp[start][ord(s[start])-97] +=1
        
        # print(queue,topo,dp)
        
        for i in adj[start]:
            for j in range(26):
                dp[i][j] = max(dp[i][j],dp[start][j])
            indeg[i]-=1
            if(indeg[i]==0):
                queue.append(i)
                
    if(count==n):
        ans = 0
        for i in range(n):
            ans = max(ans,max(dp[i]))
        return ans
    return -1
 
 
input = lambda: sys.stdin.readline().strip()
n,m = map(int,input().split())
s = input()
adj = []
for i in range(n+1):
    adj.append([])
    
for i in range(m):
    x,y = map(int,input().split())
    x-=1
    y-=1
    adj[x].append(y)
    
# print(adj)
 
ans = beautifulpath(n,adj,s)
print(ans)