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 (l > 0) {
            l -= 2 * k;
            c++;
        }
        System.err.println((c - 1) * k);
        scan.close();
    }
}