N, M = map(int, input().split())

res = 0
for i in range(30):
	if M & (1 << i):
		if N & (1 << i):
			if not i & 1:
				res |= 1 << i
		else:
			res |= 1 << i
	elif (not i & 1) and (N & (1 << i)) and not (M & (1 << i)):
	    print(-1)
	    exit()

print(res)