#include <iostream>
#include <vector>

using namespace std;

int main(){
  // input
  int L,K;
  cin >> L>>K;
  int totalcount =0;
  totalcount = (L + K -1) / K ;
  int yuucount = totalcount / 2;
  
  int ans = K * yuucount;

  if(L % (2*K)==0){
    ans -= K;
  }


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