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
		int count=0;
		if(l%(2*k)!=0){
			count=l/(2*k);
		}else if(l%(2*k)==0){
			count=l/(2*k)-1;
		}
		System.out.println(count*k);
	}
}