number = int(input())
string = input()
maxim = 0

def check(dict):
    current = 0
    boolean = False
    for k, v in dict.items():
        if v % 2 != 0:
            if boolean == True:
                return -1
            boolean = True
        current += v
    return current

for i in range(number):
    abc = {
        'a': 0,
        'b': 0,
        'c': 0,
        'd': 0,
        'e': 0,
        'f': 0,
        'g': 0,
        'h': 0,
        'i': 0,
        'j': 0,
        'k': 0,
        'l': 0,
        'm': 0,
        'n': 0,
        'o': 0,
        'p': 0,
        'q': 0,
        'r': 0,
        's': 0,
        't': 0
    }

    for j in range(number - i):
        if (j+i) < maxim:
            break
        abc[string[i+j]] += 1
        tmp = check(abc)
        if tmp != -1:
            if tmp > maxim:
                maxim = tmp

print(maxim)