def f(i, x, y):
    return x & y if i % 2 == 0 else x | y

n, m = map(int, input().split())
x = 0
for i in range(30):
    s = n >> i & 1
    t = m >> i & 1
    for v in range(2):
        if f(i, v, n >> i & 1) == m >> i & 1:
            x |= v << i
            break
    else:
        print(-1)
        break
else:
    print(x)