N,M=map(int,input().split())
ans = 0
for i in range(30):
  if (N>>i)&1 == (M>>i)&1:
    if i%2==0:
      if 1&(N>>i):
        ans |= 1 << i
    continue
  if i%2==0:
    if 1&(N>>i):
      pass
    else:
      print(-1)
      break
  else:
    if 1&(N>>i):
      pass
    else:
      ans |= 1 << i
else:
  print(ans)