N,M=map(int,input().split())
flag = True
x = 0
for i in range(30):
    a = 2**i
    n = N&a
    m = M&a
    if i%2 == 0:
        if m > 0:
            if n == 0:
                flag = False
            x += a
    else:
        if m > 0:
            if n == 0:
                x += a
        else:
            if n > 0:
                flag = False
if not flag:
    print("-1")
    exit()
print(x)