#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define fi first
#define se second

vector<int> finddivisor(int x){
    vector<int> divisor;
    for(int i=1; i*i<=x ;i++)
    {
        if(x%i==0){
            divisor.push_back(i);
        }
    }
    divisor.push_back(x);
    return divisor;
}//整数xの約数が入ったvectorを返すよ(xを含む)  

int main(){
    ll a,b;
    cin >> a >> b;
    vector<int> v=finddivisor(a+b);
    sort(v.begin(),v.end());

    for(int i=0;i < v.size();i++)
    {
        if((b+v.at(i))%a == (v.at(i)+a)%b && (b+v.at(i))%a==0 && v.at(i)!=a && b!=v.at(i)){
            cout << v.at(i) << endl;
            return 0;
        }
    }
        
    cout << -1 << endl;
}