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

res = 0

for i in range(30):
    if i & 1:
        if N >> i & 1 == 1 and M >> i & 1 == 0:
            print(-1)
            break
        elif N >> i & 1 == 0 and M >> i & 1 == 1:
            res |= 1 << i
    else:
        if N >> i & 1 == 0 and M >> i & 1 == 1:
            print(-1)
            break
        elif M >> i & 1 == 1:
            res |= 1 << i
else:
    print(res)