#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll a,b;cin>>a>>b;
    ll u=b-a;
  if((u+a)%b==0&&(u+b)%a==0&&(a+b)%u==0){
    cout<<u<<endl;
    return 0;
  }
  u=2*b-a;
  if((u+a)%b==0&&(u+b)%a==0&&(a+b)%u==0){
    cout<<u<<endl;
    return 0;
  }
  cout<<-1<<endl;
}