import java.util.Scanner;

public class No63 {

	public void run(){
		Scanner sc = new Scanner(System.in);
		int L = sc.nextInt();
		int K = sc.nextInt();

		System.out.println((L-1)/(2*K)*K);
		
	}
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		new No63().run();
	}

}