#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

typedef long long ll;

int main(){
    int A,B;
    cin >> A >> B;
    vector<ll> v;
    for(ll C=2;C*C<=A+B;C++){
        if((A+B)%C==0){
            v.push_back(C);
            v.push_back((A+B)/C);
        }
    }
    sort(v.begin(),v.end());
    for(ll i=0;i<v.size();i++){
        if((A+v[i])%B==0&&(B+v[i])%A==0){
            cout << v[i] << endl;
            return 0;
        }
    }
    cout << -1 << endl;
}