import java.util.Scanner;

class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int l = scan.nextInt();
        int k = scan.nextInt();
        int c = 0;
        while (true) {
            l -= 2 * k;
            if (l <= 0) {
                break;
            } else {
                c++;
            }
        }
        System.out.println(c * k);
        scan.close();
    }
}