#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> lpair;
const ll MOD = 1e9 + 7;
const ll INF = 1e18;
#define rep(i,m,n) for(ll i = (m); i < (n); i++)
#define rrep(i,m,n) for(ll i = (m); i >= (n); i--)
#define print(x) cout << (x) << endl;
#define print2(x,y) cout << (x) << " " << (y) << endl;
#define printa(x,n) for(ll i = 0; i < n; i++){ cout << (x[i]) << " ";} cout<<endl;
inline vector<ll> divisor(ll M){ //約数の全列挙
    vector<ll> dd;
    for(ll i = 1; i*i <= M; i++){
        if(M % i == 0){
            dd.push_back(i);
            if(i * i != M){
                dd.push_back(M/i);
            }
        }
    }
    sort(dd.begin(), dd.end());
    return dd;
}
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll A,B;
    cin >> A >> B;
    vector<ll> dd = divisor(A+B);
    rep(i,0,dd.size()){
        if((A + dd[i]) % B == 0 && (B + dd[i]) % A == 0 && A != dd[i] && B != dd[i]){
            print(dd[i]);
            return 0;
        }

    }
    print(-1);
    


}