n, m = list(map(int, input().split()))
ans = 0
for i in range(30):
	n0 = n >> i & 1
	n1 = m >> i & 1
	if i % 2:
		if n0 and not n1: exit(print(-1))
		elif n1 and not n0:
			ans ^= 1 << i
	else:
		if n1 and not n0: exit(print(-1))
		elif n1: ans ^= 1 << i
print(ans)