import java.util.Scanner;

public class No63 {

    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int pocky = s.nextInt();
        int bite = s.nextInt();
        int eachBite = bite * 2;
        int biteTime = pocky / eachBite;
        if (pocky % eachBite == 0) {
            biteTime--;
        }
        System.out.println(biteTime * bite);
    }
}