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();

		if(L/2<=K) {
			System.out.println(0);
		}else {
			System.out.println(L/K/2*K);
		}
	}

}