import java.util.Scanner;

public class Main63 {
	public static void main(String[] args) {
		Main63 p = new Main63();
	}

	public Main63(){
		Scanner sc = new Scanner(System.in);
		int l = sc.nextInt();
		int k = sc.nextInt();
		System.out.println((l-1)/(2*k)*k);
	}

}