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

class Main
{
	public static void main (String[] args) 
	{
		// 入力値の読み込み
		Scanner sc = new Scanner(System.in);
		int  l = sc.nextInt();
		int  k = sc.nextInt();
		int count=0;
		
		while(l > k*2){
			l -= k*2;
			count +=1;
		}
		
		System.out.println(k*count);
		
	}
}