import java.util.Scanner;

public class No63 {

	public void run(){
		Scanner sc = new Scanner(System.in);
		int L = sc.nextInt();
		int K = sc.nextInt();
		int a=L-2*K;
		int count;
		
		if(a >0)
			count=1;
		else
			count=0;
		
		while( a > 0){
			 a=a-2*K;
			 if(a<0)
				 break;
			 count++;
		}
		
		System.out.println(count*K);
		
	}
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		new No63().run();
	}

}