import java.io.*;
import java.util.*;

class Main{
  public static void main(String[] args) throws IOException{

    Scanner scan=new Scanner(System.in);
    int l=Integer.parseInt(scan.next());
    int k=Integer.parseInt(scan.next());

    int count=0;
    while(l>2*k){
      l-=2*k;
      count++;
    }
    
    int yuu=k*count;
    System.out.println(yuu);
  }
}