public class Hello
{
    public static void Main()
    {
        //var sr = new System.IO.StreamReader("test.txt");
        //System.Console.SetIn(sr);

        string line = System.Console.ReadLine().Trim();
        string[] Data = line.Split(' ');
        int L = int.Parse(Data[0]);
        int K = int.Parse(Data[1]);

        double x = (double)(L) /(double)(2 * K);
        

        if (x <= 1)
        {
            System.Console.WriteLine(0);
            return;
        }
        else if(x == (int)x)
        {
            System.Console.WriteLine((int)x * K-K);
            return;
        }
        else
        {
            System.Console.WriteLine((int)x * K);
            return;

        }
        
    }
}