using System;
using System.Linq;

public class Program
{
    public static void Main(string[] args)
    {
        int l, k;
        var data = Console.ReadLine().Split(' ').Select(x => int.Parse(x)).ToArray();
        l = data[0];
        k = data[1];
        var count = 0;

        while (l > 0)
        {
            l -= k*2;
            count++;
        }
        Console.WriteLine((count-1) * k);
    }
}