A, B = map(int, input().split())

D = []
for C in range(1, int((A + B) ** 0.5) + 100):
	if (A + B) % C == 0:
		D.append(C)
		D.append((A + B) // C)

for C in sorted(D):
	if A != C and B != C and (A + B) % C == (B + C) % A == (C + A) % B == 0:
		print(C)
		break
else:
	print(-1)