using System;

public class Paiza
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var L = int.Parse(line[0]);
        var k = int.Parse(line[1]);
        var ans = L / (2 * k);
        if (L % (2 * k) == 0) ans--;
        Console.WriteLine(ans * k);
    }
}