using System;
public class Program
{
    public static void Main()
    {
        string[] pass = (Console.ReadLine() ?? string.Empty).Trim().Split(' ');

        int po = int.Parse(pass[0]);
        int mm = int.Parse(pass[1]);
        if(po <= mm*2)
        {
            Console.WriteLine("0");
            return;
        }
        int aaa = po / (mm * 2);
        if(po % (mm * 2) == 0)
        {
            aaa -= mm;
        }
        Console.WriteLine(aaa*mm);
    }
}