#include<bits/stdc++.h>
using namespace std;

#define rep(i,n) for(ll i=0;i<(n);++i)
using ll = long long;
using pll = pair<ll,ll>;
constexpr ll INF = (1LL<<60);
constexpr ll MOD = (1e9+7);
//constexpr ll MOD = (998244353);
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}return 0;}
void dump(){cout<<endl;}
template<class T,class... Ts> void dump(T&& h, Ts&&... t){cout<<h<<", ";dump(forward<Ts>(t)...);}
template<class T> istream &operator>>(istream&is,vector<T>&v){for(auto &elemnt:v)is>>elemnt;return is;}
template<class T,class U> istream &operator>>(istream&is,pair<T,U>&p){is>>p.first>>p.second;return is;}
template<class T>vector<T> make_vector(size_t a){return vector<T>(a);}
template<class T, class... Ts>auto make_vector(size_t a, Ts... ts){return vector<decltype(make_vector<T>(ts...))>(a, make_vector<T>(ts...));}

void solve1();void solve2();
int main(){
  solve1();
  return 0;
}
std::vector<ll> divisor(ll n){
  std::vector<ll> div;
  for(ll p=1;p*p<=n;++p){
    if(n%p==0){
     div.push_back(p);
     if(p*p!=n)div.push_back(n/p);
    }
  }
  std::sort(div.begin(),div.end());
  return div;
}
void solve1(){

  ll a,b;
  cin>>a>>b;

  ll t = a+b;
  auto div = divisor(t);

  ll ans = -1;
  for(auto c:div){
    bool x,y,z;
    x = (a+b)%c==0;
    y = (a+c)%b==0;
    z = (b+c)%a==0;
    if(x and y and z)ans=c;
  }
  cout<<(ans)<<endl;


}