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

int main(){
	int n,m; cin >> n >> m;
	int r = 0;
	for (int i(0);i < 30;++i){
		if (i%2==0&&(m>>i)%2) r |= (1<<i);
		else if (i%2==1&&(m>>i)%2&&(n>>i)%2==0) r |= (1<<i);
		if (i%2==0&&((r>>i)&(n>>i))%2!=(m>>i)%2) (cout<<-1<<endl),exit(0);
		if (i%2==1&&((r>>i)|(n>>i))%2!=(m>>i)%2) (cout<<-1<<endl),exit(0);
	}
	cout << r << endl;
}