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;
        }

        while(po >= mm * 2)
        {
            you += int.Parse(pass[1]);
            po = po - mm * 2;
        }
        Console.WriteLine(you);
    }
}