#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}

//INSERT ABOVE HERE
signed main(){
  Int a,b;
  cin>>a>>b;
  Int d=a+b;
  auto check=[](Int a,Int b,Int c){
               if(a==b||b==c||c==a) return;
               if((a+b)%c) return;
               if((b+c)%a) return;
               if((c+a)%b) return;
               cout<<c<<endl;
               exit(0);
             };
  vector<Int> vs;
  for(Int i=1;i*i<=d;i++){
    if(d%i) continue;
    vs.emplace_back(i);
    vs.emplace_back(d/i);
  }
  sort(vs.begin(),vs.end());
  for(Int c:vs) check(a,b,c);
  cout<<-1<<endl;
  return 0;
}