import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final long L = sc.nextLong();
		final int K = sc.nextInt();
		
		System.out.println((L - 1) / (2 * K) * K);
		
	}
	
}