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 i & 1 and N & (1 << i) and not M & (1 << i):
	    print(-1)
	    exit()

print(res)