#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,m,n) for(int i=m; i<n; ++i)
#define repl(i,m,n) for(ll i=m; i<n; ++i)

int main(){
    ll A, B;
    cin >> A >> B;

    vector<ll> divisor;
    for(ll i=1LL; i*i <= A+B; ++i){
        if((A+B)%i != 0) continue;
        divisor.push_back(i);
        if(i*i != A+B) divisor.push_back((A+B)/i);
    }
    sort(divisor.begin(), divisor.end());

    ll ans = -1LL;
    for(ll d : divisor){
        if(d == A || d == B) continue;
        if((d + A)%B == 0 && (d + B)%A == 0){
            ans = d;
            break;
        }
    }
    cout << ans << endl;

    return 0;
}