import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int l = sc.nextInt();
		int k = sc.nextInt();
		int count = 0, ans = 0;
		while (l > 2 * k) {
			l -= 2 * k;
			count++;
		}
		ans = count * k;
		System.out.println(ans);
	}
}