public class Hello
{
    public static void Main()
    {
        string line = System.Console.ReadLine().Trim();
        string[] Data = line.Split(' ');
        int L = int.Parse(Data[0]);
        int K = int.Parse(Data[1]);

        int x = L / (2 * K);

        if (x <= 1)
        {
            System.Console.WriteLine(0);
            return;
        }

        System.Console.WriteLine(x * K);
        return;
    }
}