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



int main(){
	int A,B;
	cin>>A>>B;
	vector<int> V;
	for(long long i = 1;i*i<=(long long)A+B;i++){
		if((A+B)%i==0){
			V.push_back(i);
			V.push_back((A+B)/i);
		}
	}
	
	sort(V.begin(),V.end());
	
	for(int i=0;i<V.size();i++){
		if((A+V[i])%B+(B+V[i])%A==0&&V[i]!=A&&V[i]!=B){
			cout<<V[i]<<endl;
			return 0;
		}
	}
	
	cout<<-1<<endl;
		
	
	return 0;
}