import java.util.*;
public class Pokky{
	public static void main(String[] arg){
		Scanner sc = new Scanner(System.in);
		int l,k,temp,res;
		l = sc.nextInt();
		k = sc.nextInt();
		temp = (l-1)/(2*k);
		res = temp * k;
		System.out.println(res);
	}
}