#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001LL

int main(){
	int n,m;
	cin>>n>>m;
	int ans = 0;
	rep(i,30){
		rep(j,2){
			if(i%2==0){
				if((j & ((n>>i)&1)) == ((m>>i) &1)){
					ans += j<<i;
					break;
				}
				if(j==1){
					cout<<-1<<endl;
					return 0;
				}
			}
			else{
				if((j | ((n>>i)&1)) == ((m>>i) &1)){
					ans += j<<i;
					break;
				}
				if(j==1){
					cout<<-1<<endl;
					return 0;
				}
			}
		}
	}
	cout<<ans<<endl;
	return 0;
}