n,m = list(map(int,input().split()))
n = list(map(int,list(bin(n)[2:])))
m = list(map(int,list(bin(m)[2:])))
c = 0
ans = 0

while len(n) and len(m):
    c += 1
    x,y = n.pop(),m.pop()
    
    if c%2 == 0:
        if y == x:
            continue
        elif y == 0 and x == 1:
            print(-1)
            exit()
        else:
            ans |= 1<<c
    
    else:
        if y == x:
            if y == 1:
                ans |= 1<<c
            continue
        elif y == 1 and x == 0:
            print(-1)
            exit()

while len(m):
    c += 1
    x = m.pop()
    
    if c%2 == 1:
        if x == 1:
            print(-1)
            exit()
    elif x == 1:
        ans |= 1<<c

while len(n):
    c += 1
    x = n.pop()
    
    if c%2 == 0:
        if x == 1:
            print(-1)
            exit()

print(ans>>1)