import sys
input = sys.stdin.readline

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

ANS=0

for i in range(31):
    if i%2==0:
        if M & (1<<i)!=0:
            if N & (1<<i) != 0:
                ANS|=(1<<i)
            else:
                print(-1)
                exit()
    else:
        if M & (1<<i)!=0:
            if N & (1<<i) == 0:
                ANS|=(1<<i)
        else:
            if N & (1<<i) != 0:
                print(-1)
                exit()

print(ANS)