import java.util.Scanner;


public class Main_yukicoder63 {

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

        int l = sc.nextInt();
        int k = sc.nextInt();

        if (l % (2 * k) == 0) {
        	System.out.println((l / (2 * k) - 1) * k);
        } else {
        	System.out.println(l / (2 * k) * k);
        }
        
        
        sc.close();
    }
}