import java.util.*;
import java.util.stream.*;
public class Main{
	public static void main(String... args){
		Scanner sc=new Scanner(System.in);
		int l=sc.nextInt();
		int k=sc.nextInt();
		if(l%(2*k)==0){
			put((l/(2*k)-1)*k);
		}else{
			put((l/(2*k))*k);
		}
		
	}
	
	public static void print(Object object){
        System.out.print(object);
    }
    public static void put(Object object) {
        System.out.println(object);
    }
    public static void put(){
        System.out.println();
    }
 
    public static void print(String format,Object... args){
        System.out.print(String.format(format,args));
    }
    public static void put(String format,Object... args) {
        System.out.println(String.format(format,args));
    }
}