#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll mod=1e9+7;

ll A,B;

int main(){ 

cin.tie(0);
ios::sync_with_stdio(false);

cin>>A>>B;

ll AB=A+B;
for(ll C=1; C<=sqrt(AB); C++){
	if(AB%C==0){
		if(A==C||B==C) continue;
		ll AC=A+C;
		ll BC=B+C;
		if((AC%B==0)&&(BC%A==0)){
			cout<<C<<endl;
			return 0;
		}
	}
}

cout<<-1<<endl;

}