import re
import itertools

def alternate(s):
    maxLen = 0
    unique_char = set(s)
    n = len(unique_char)
    pattern = re.compile(r'^(\w)(?!\1)(\w)(\1\2)*\1?$')
    
    if (n < 2):
        maxLen = 0
    else:
        for comb in itertools.combinations(unique_char, n-2):
            x = s
            
            for i in comb:
                x = x.replace(i, "")
            
            if (pattern.match(x)):
                maxLen = max(maxLen, len(x))
    
    return maxLen

length = int(input())
s = input()

result = alternate(s)

print(result)