using System;
using System.Linq;

namespace TestConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            var inputs = Console.ReadLine().Split().Select(i => int.Parse(i));

            Console.WriteLine(((inputs.ElementAt(0) -1) / (inputs.ElementAt(1) * 2)) * inputs.ElementAt(1));
        }
    }
}