package yukicoder063;
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		int l=sc.nextInt();//ポッキーの長さ　Lmm
		int k=sc.nextInt();//お互いが１回にかじるポッキーの長さ Kmm
		System.out.println((l-1)/(2*k)*k);
	}
}