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.

Input

The first line contains two positive integers n, m (1 ≤ n, m ≤ 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 ≤ x, y ≤ 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.

Output

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

Examples

Input

5 4
abaca
1 2
1 3
3 4
4 5


Output

3


Input

6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4


Output

-1


Input

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


Output

4

Note

In the first sample, the path with largest value is 1 → 3 → 4 → 5. The value is 3 because the letter 'a' appears 3 times.
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)
    
    
    
    
    
    
    
    
    
    
    