#include <bits/stdc++.h>
#include <atcoder/modint.hpp>
using namespace std;
 
using i32 = int;
using i64 = long long;
using i128 = __int128_t;
using f64 = double;
using p2 = pair<i64, i64>;
using p3 = tuple<i64, i64, i64>;
using mint = atcoder::modint998244353;
constexpr i64 inf = 1e18;
 
void _main();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(18);
  _main();
}

void _main() {
  i64 n, m;
  cin >> n >> m;
  i64 x = 0;
  for (i64 i = 0; i < 30; i++) {
    i64 a = n >> i & 1;
    i64 b = m >> i & 1;
    if (i % 2 == 0) {
      if (b == 1 && a == 0) {
        cout << "-1\n";
        return;
      }
      if (b == 1) x += 1 << i;
    } else {
      if (b == 0 && a == 1) {
        cout << "-1\n";
        return;
      }
      if (b == 1 && a == 0) x += 1 << i;
    }
  }
  cout << x << "\n";
}