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

ans = 0
for i in range(30):
    if(i%2 == 0):
        n = N>>i & 1
        m = M>>i & 1
        if(n == 0 and m == 1):
            print(-1)
            exit()
        ans |= 1<<i if m == 1 else 0
    else:
        n = N>>i & 1
        m = M>>i & 1
        if(n == 1 and m == 0):
            print(-1)
            exit()
        ans |= 1<<i if n == 0 and m == 1 else 0
print(ans)