import java.util.Scanner;

public class No63 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int l = sc.nextInt();
        int k = sc.nextInt();

        int remain = l;
        int cnt = 0;

        while (remain > 0) {
            remain -= 2 * k;
            cnt++;
        }

        int result = k * (cnt - 1);
        System.out.println(result);

        sc.close();
    }

}