import sys
# input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n, m = na()

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