using System;
using System.Collections.Generic;
namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
#if LocalDebug
            var exStdIn = new System.IO.StreamReader("stdin.txt");
            System.Console.SetIn(exStdIn);
#endif
            var Input = GetList();
            var Length = int.Parse(Input[0]);
            var Eat = int.Parse(Input[1]);
            var time = 0;

            while (true)
            {
                if (Length <= (Eat * 2))
                {
                    break;
                }
                Length -= (Eat * 2);
                time++;
            }

            Console.WriteLine(Eat * time);


#if LocalDebug
            System.Console.ReadKey();
#endif
        }




        static int GetData()
        {
            return int.Parse(Console.ReadLine());
        }

        static string[] GetList()
        {
            return Console.ReadLine().Split(' ');
        }



    }
}