package yukicoder;

import java.util.Scanner;

public class N63 {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int l = sc.nextInt();
		int k = sc.nextInt();
		int y = 0;
		while(true){
			l -= 2 * k;
			if(l <= 0){
				System.out.println(y);
				return;
			}
			y += k;
		}
	}
}