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)))
aSA9IGlucHV0KCkKd2hpbGUgKCBpICE9IC0xKToKCXBhY2tzID0gW10KCWZvciBqIGluIHJhbmdlKGkpOgoJCXBhY2tzLmluc2VydChpbnB1dCkKCXN1bSA9IHJlZHVjZShsYW1iZGEgYSwgYjogYStiLCBwYWNrcykKCWlmIHN1bSAlIGkgIT0gMDoKCQlwcmludCAiLTEiCgllbHNlOgoJCWRlc2lyZWQgPSBzdW0gLyBpCgkJcHJpbnQgcmVkdWNlKGxhbWJkYSBhLCBiOiBhK2IsIG1hcChsYW1iZGEgeDogZGVzaXJlZCAtIHgsIGZpbHRlcihsYW1iZGEgeDogeCA8IGRlc2lyZWQscGFja3MpKSkKCQoK