class Program
{
    static void Main(string[] args)
    {
        string[] inputNum = Console.ReadLine()!.Split(' ');
        int allLength = int.Parse(inputNum[0]);
        int once = int.Parse(inputNum[1]);

        int half = allLength / 2;
        int eatCount = half / once;

        if (eatCount == 1)
        {
            Console.WriteLine(0);
        }
        else
        {
            int ans = eatCount * once;
            Console.WriteLine(ans);
        }
    }
}