import java.util.Scanner;

public class Main{
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int L = scan.nextInt();
		int K = scan.nextInt();
		scan.close();
		int k1 = L / (2 * K);
		int k2 = L % (2 * K);
		if(k2 == 0) {
			k1 = k1 - 1;
		}
		System.out.println(k1 * K);

	}
}