using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        int[] array = Console.ReadLine().Split(' ').Select(a => int.Parse(a)).ToArray();
        int mm = 0;
        int width = array[0];

        while (true)
        {
            width -= array[1] * 2;

            if (width <= 0)
            {
                break;
            }

            mm += array[1];
        }

        Console.WriteLine(mm);
    }
}