#include <bits/stdc++.h>
using namespace std;
int main( ){
   int L,K;
   cin >> L >> K ;
   if(L <= 2*K){
       cout << 0 << endl;
   }
   else if(L % (2*K) == 0){
       cout << (L/(2*K)-1)*K << endl;
   }
   else{
       cout << L/(2*K)*K << endl;
   }
   
}