#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)
#define repe(i,a)for(auto &i:a)

void in(){} template<typename Head,typename... Tail> void in(Head&& head,Tail&&... tail){cin>>head;in(forward<Tail>(tail)...);}

vector<ll> divisor(ll n){
  vector<ll> res;
  for(ll i=1;i*i<=n;i++){
    if(n%i==0){
      res.push_back(i);
      if(i!=n/i) res.push_back(n/i);
    }
  }
  sort(res.begin(),res.end());
  return res;
}

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  ll a,b;
  in(a,b);
  auto ed=divisor(a+b);
  repe(c,ed){
		if(a==c)continue;
		if(b==c)continue;
		if((a+c)%b!=0)continue;
		if((b+c)%a!=0)continue;
    cout<<c<<endl;
    return 0;
  }
  cout<<-1<<endl;
}