def getlist():
	return list(map(int, input().split()))

A, B = getlist()
A, B = sorted([A, B])
if B % A != 0:
	if B % (B - A) == 0 and int(B // (B - A)) == 3:
		print(B - A)
	else:
		print(-1)
else:
	if int(B // A) == 2:
		print(A * 3)
	elif int(B // A) == 3:
		print(A * 2)
	else:
		print(-1)