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

int main() {
    int m, n;
    cin >> n >> m;

    bitset<30> bm(m);
    bitset<30> bn(n);
    
    bool istrue = 1;
    for(int i = 0; i<30; i++){
        if((i%2 == 0 && (bm[i] == 1 && bn[i] == 0)) || (i%2!= 0 &&(bn[i] == 1 && bm[i] == 0))){
            istrue = 0;
            break;
        }}
    if(istrue == 0) cout << -1;
    else{
    bitset<30> ans;
    for(int i = 0; i<30;i++){
        if(i%2 == 0) ans[i] = bm[i];
        if(i%2!= 0){
            if(bn[i] == 1) ans[i] = 0;
            else ans[i] = bm[i];
        }
    }
    cout << ans.to_ulong();}
}