import java.util.*;
public class pocky{
	public static void main(String[] args){
		Scanner scan = new Scanner(System.in);
		int L = scan.nextInt();
		int K = scan.nextInt();
		System.out.println(eat(L,K));
	}
	public static int eat(int l,int k){
		return (l-1)/(k*2)*k; 
	}
}