import java.util.*;

public class Main {
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int length=sc.nextInt();
        int eat=sc.nextInt();
        
        int count=0;
        
        while(length>eat*2){
            length-=eat*2;
            count++;
        }
    
        System.out.println(eat*count);
    }
}