#include <iostream>
#include <vector>
using namespace std;
typedef long long LL;
int main(int argc, char* argv[])
{
	LL A,B;
	cin>>A>>B;
	LL AB=A+B;
	vector<LL> C;
	LL S=A-B;
	if (S<0){
		S=-S;
	}
	for (LL i=S;i<=AB;i++){
		if (AB%i==0){
			C.push_back(i);
		}
	}
	vector<LL> C1;
	for (int i=0;i<C.size();i++){
		if ((A+C[i])%B==0 && (B+C[i])%A==0 && A!=C[i] && B!=C[i]){
			C1.push_back(C[i]);
		}
	}
	if (C1.size()==0){
		cout<<-1<<endl;
	}else{
		cout<<C1[0]<<endl;
	}

	return 0;
}