#ifndef INCLUDED_MAIN
#define INCLUDED_MAIN

#include __FILE__



int main(void){
    ll n, m; cin >> n >> m;
    ll ans = 0;
    ll X = max(n,m), Y = 1;
    ll N = n, M = m;
    while(X > 0){
        if((N&1) && (M&1)) ans += Y;
        else if(!(N&1) && (M&1)) {ans = -1; break;}
        X>>=2;
        Y<<=2;
        M>>=2;
        N>>=2;

    }
    X = max(n,m); Y = 1;
    N = n; M = m;
    while(X > 0){
        if((N&2) && !(M&2)) {ans = -1; break;}
        else if(!(N&2) && (M&2)) ans += Y*2;
        X>>=2;
        Y<<=2;
        M>>=2;
        N>>=2;
    }
    cout << ans << "\n";
}



#else
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;
#define rep(i,n) for(i=0;i<(n);i++)
#define all(a) a.begin(), a.end()
#define min(a,b) (a<b?a:b)
#define max(a,b) (a>b?a:b)


#endif