import java.util.Scanner;

public class Main{
    public static  void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        int l = sc.nextInt();
        int k = sc.nextInt();
        int count = 0;
        
        while(l > (2 * k)){
            l = l - k * 2;
            count++;
        }
        
        System.out.println(k * count);
    }
}