import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        String[] line;
        line=sc.nextLine().split(" ");
        int tmp=Integer.parseInt(line[0]);
        int tmp1=Integer.parseInt(line[1]);
        tmp=(tmp-1)/2/tmp1*tmp1;
        System.out.println(tmp);
    }
}