days = int(input())
mugs = input()

brands = {}

def is_valid():
    oddCount = 0
    for brand in brands.values():
        if brand % 2 != 0:
            oddCount += 1
        if oddCount > 1:
            return False
    return True

def count_brands(string):
    for mug in string:
        if mug not in brands:
            brands.update({mug: 1})
        else:
            brands.update({mug: brands[mug] + 1})

max = 0
for i in range(len(mugs)):
    for l in range(len(mugs)):
        selected = mugs[i:l + 1]
        count_brands(selected)
        if is_valid():
            if len(selected) > max:
                max = len(selected)
        brands.clear()

print(max)