using System;
public class Program
{
    public static void Main()
    {
        //int num = int.Parse(Console.ReadLine() ?? string.Empty);
        string[] pass = (Console.ReadLine() ?? string.Empty).Trim().Split(' ');
        //string a = Console.ReadLine() ?? string.Empty;
        //string b = Console.ReadLine() ?? string.Empty;


        int po = int.Parse(pass[0]);
        int mm = int.Parse(pass[1]);
        //int you = 0;
        if(po <= mm*2)
        {
            Console.WriteLine("0");
            return;
        }
        int aaa = po / (mm * 2);
        if(po % (mm * 2) == 0)
        {
            aaa -= mm;
        }
        //while(po >= mm * 2)
        //{
        //    you += int.Parse(pass[1]);
        //    po -= mm * 2;
        //}
        //Console.WriteLine(you);
        Console.WriteLine(aaa*mm);
    }
}