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

ll A,B,C;

int main(){ 

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

cin>>A>>B;

ll AB=A+B;

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

cout<<-1<<endl;

}