c = raw_input()
line = [ c for c in raw_input() ]
def getlen(line):
	line = sorted(line)
	#print(line)
	rev={}
	for c in line:
		if not (c in rev):
			rev[c] = 1
		else:
			rev[c] = rev[c] + 1
	c = 0
	did = False
	#print(line)
	#print(rev)
	for k in rev:
		v = rev[k]
		if v % 2 == 1:
			if not did:
				did = True
				c = c + 1
		v = v >> 1
		v = v << 1
		c = c + v
	return c
#print(getlen(line[1:]))
def process(line):
	for i in range(0, len(line)):
		for j in range(len(line), 0, -1):
			o = line[i:j]
			if getlen(o) == len(o):
				return len(o)
	return 0
print(process(line))