#ifdef NACHIA
#define _GLIBCXX_DEBUG
#else
// disable assert
#define NDEBUG
#endif
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
using namespace std;
using ll = long long;
const ll INF = 1ll << 60;
#define REP(i,n) for(ll i=0; i<ll(n); i++)
template <class T> using V = vector<T>;
template <class A, class B> void chmax(A& l, const B& r){ if(l < r) l = r; }
template <class A, class B> void chmin(A& l, const B& r){ if(r < l) l = r; }

void testcase(){
  ll N, M; cin >> N >> M;
  ll A = (1ll << 30) - 1, B = 0;
  REP(j,30){
    ll a = N & (1ll << j);
    ll b = M & (1ll << j);
    if(j%2 == 0){
      if(((0ll << j) & a) == b) A -= 1ll << j;
      if(((1ll << j) & a) == b) B += 1ll << j;
    } else {
      if(((0ll << j) | a) == b) A -= 1ll << j;
      if(((1ll << j) | a) == b) B += 1ll << j;
    }
  }
  if((B&A) == A){ cout << A << "\n"; } else cout << "-1\n";
}

int main(){
  cin.tie(0)->sync_with_stdio(0);
  testcase();
  return 0;
}