using System;
using System.Linq;

class Sample
{
    public static void Main()
    {
        var pokky = Console.ReadLine().Trim().Split(" ")
            .Select(int.Parse).ToList();
        int num = pokky[0] / (2 * pokky[1]);
        Console.WriteLine(num==0?(num-1)*pokky[1]: pokky[1]*num);
    }
}