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

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N,M; cin >> N >> M;
    int answer = 0;
    for(int d=0; d<30; d++){
        int v1 = (N>>d)&1,v2 = (M>>d)&1;
        if(d%2){
            if(v2 == 1){
                if(v1 == 0) answer += 1<<d;
            }
            else{
                if(v1){cout << -1 << endl; return 0;}
            }
        }
        else{
            if(v2 == 1){
                if(v1 == 1) answer += 1<<d;
                else {cout << -1 << endl; return 0;}
            }
        }
    }

    cout << answer << endl;
}