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 % (K * 2) == 0) {
			System.out.println((L / K / 2 - 1) * K);
		} else {
			System.out.println(L / K / 2 * K);
		}
	}

}