#include<iostream>
#include<algorithm>
#include<stdio.h>
#include<stdlib.h>
#include<vector>
#include<string>
#include<sstream>
using namespace std;


int main(){

  int l,k;
  cin >> l >> k;

  int cnt=0;
  while(l>=0){
    if( l-k*2 <= 0 ) break;
    l -= k*2;
    cnt += k;
  }

  cout << cnt << endl;

  return 0;
}