i = input()
while ( i != -1):
	packs = []
	for j in range(i):
		packs.insert(input)
	sum = reduce(lambda a, b: a+b, packs)
	if sum % i != 0:
		print "-1"
	else:
		desired = sum / i
		print reduce(lambda a, b: a+b, map(lambda x: desired - x, filter(lambda x: x < desired,packs)))