#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    ll a,b;cin>>a>>b;
    for(ll i=1;i*i<=a+b;i++)if((a+b)%i==0){
        ll x=i;
        ll c=(a+b)/x;
        if((b+c)%a==0 and (c+a)%b==0){
            cout<<c<<endl;
            return 0;
        }

        x=(a+b)/i;
        c=(a+b)/x;
        if((b+c)%a==0 and (c+a)%b==0){
            cout<<c<<endl;
            return 0;
        }
    }
    cout<<-1<<endl;
    return 0;
}