#include <iostream>
#include <vector>

using namespace std;

int main(){
  // input
  long long L,K;
  cin >> L>>K;
  int totalcount =0;
  

  totalcount = (L / K);
  int yuucount;
  if(L % (2*K) == 0){
    yuucount = (totalcount/2)-1;
  }else {
    yuucount = totalcount / 2;
  } 
  int ans = K * yuucount;


  cout << ans << endl;
  // output 
}