N, M = list(map(int, input().split()))
ans = 0
for i in range(30):
    nb = (N & (1 << i)) >> i
    mb = (M & (1 << i)) >> i
    if i % 2 == 0:
        if nb == 0 and mb == 1:
            print(-1)
            exit()
        elif nb == 1 and mb == 1:
            ans += (1 << i)
    else:
        if mb == 0 and nb == 1:
            print(-1)
            exit()
        elif mb == 1 and nb == 0:
            ans += (1 << i)
print(ans)