def getIntervals(lstLetters):
	i = 0
	slovnik ={}
	for i in range(len(lstLetters)):
		if slovnik.get(lst[i]) == None:
			slovnik[lst[i]] = 1
		else:
			slovnik[lst[i]] = slovnik[lst[i]] +1


	length = 0
	unpairCount = 0

	for key, value in slovnik.items(): 
		if(value%2 ==0):
			length += value
		else:
			unpairCount += 1
			if unpairCount > 1:
				break
			length += value 

	if unpairCount > 1:
		return False
	

	
	return length


a = input()
if(int(a) <= 300000 and int(a) >1):

	lst = []
	lst = input()
	lst = lst[:int(a)]

	maximum = 0
	lenA = int(a) 
	k = 0

	for k in range(lenA):

		sizeOfRange = lenA - k
		j = 0
		start =0
		for j in range(lenA):

			pocet = getIntervals(lst[start:sizeOfRange])
			if pocet > maximum:
		
				maximum = pocet
			if start + sizeOfRange < lenA:

				start += 1
			else:
				break
		if maximum > 0:
			break
		lenA = lenA - 1

	print(maximum)