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

ll A,B;
ll ans;

int main(){ 

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

cin>>A>>B;

ll AB=A+B;
ll ans=mod;
for(ll C=1; C<=sqrt(AB); C++){
	if(AB%C==0){
		ll AC=A+C;
		ll BC=B+C;
		if((AC%B==0)&&(BC%A==0)){
			if(A!=C&&B!=C) ans=min(ans,C);
		}
		ll D=AB/C;
		ll AD=A+D;
		ll BD=B+D;
		if((AD%B==0)&&(BD%A==0)){
			if(A!=D&&B!=D) ans=min(ans,D);
		}
	}
}

if(ans!=mod) cout<<ans<<endl;
else cout<<-1<<endl;

}