#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll mod=1000000007;
ll inf=10099999999999990;
int main(){
    ll a,b;
    cin >> a >> b;
    ll x=a+b;
    vector<ll>y;
    for (ll i = 1; i*i <=x; i++)
    {
        if (x%i==0)
        {
            y.push_back(i);
            y.push_back(x/i);
        }
    }
    ll ans=inf;
    for (ll i = 0; i < y.size(); i++)
    {
        if (y[i]==a||y[i]==b)
        {
            continue;
        }
        if ((y[i]+a)%b==0&&(y[i]+b)%a==0)
        {
            ans=min(ans,y[i]);
        }
    }
    if (ans==inf)
    {
        cout << -1 << endl;
    }else{
        cout << ans << endl;
    }
}