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
	#print("Slovnik: ",slovnik)

	length = 0
	unpairCount = 0

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

	if unpairCount > 1:
		return False
	
	#print("Dlzka: ",length)
	
	return length

#Tuto
a = input("Zadejte pocet pismen: ")
lst = []
lst = input("Zadejte pismena: ")

lst = lst[:int(a)]
#print("LIST:",lst)
#print("pocet slov:", a)


maximum = 0
lenA = int(a) 
k = 0
#print(a)
for k in range(lenA):
	#print("1 fpr")
	sizeOfRange = lenA - k
	j = 0
	start =0
	for j in range(lenA):
		#print("2 fpr")
		pocet = getIntervals(lst[start:sizeOfRange])
		if pocet > maximum:
			#print("pocet > max")		
			maximum = pocet
		if start + sizeOfRange < lenA:
			#print("start++")
			start += 1
		else:
			#print("break")
			break

	lenA = lenA - 1

print(maximum)