#include<bits/stdc++.h>
using namespace std;
int main(){
    int a,b;cin >> a >> b;
    vector<int> v;
    int num=a+b;
    for(int i=1;i*i<=num;i++){
        if(num%i==0){
            v.push_back(i);
            if(i*i!=num){
                v.push_back(num/i);
            }
        }
    }
    sort(v.begin(),v.end());
    for(int i=0;i<v.size();i++){
        if(a+v[i]>=b&&b+v[i]>=a&&a!=v[i]&&b!=v[i]){
            if((a+v[i])%b==0&&(b+v[i])%a==0){
                cout << v[i] << endl;
                return 0;
            }
        }
    }
    cout << -1 << endl;
}