package test.demo;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Yukicoder63 {

	public static void main(String[] args) throws IOException {
		// TODO 自動生成されたメソッド・スタブ
		String[]str=input();
		int L=Integer.parseInt(str[0]);
		int K=Integer.parseInt(str[1]);
		int count=0;
		
		while(true) {
			L=L-2*K;
			if(L<=0) {
				System.out.println(count);
				return;
			}
			count+=K;
		}
		

	}

	public static  String[] input() throws IOException {
		BufferedReader br =new BufferedReader(new InputStreamReader(System.in));
		
		String str=br.readLine();
		String[]str2=str.split(" ");
		
		return str2;
	}
}