using System;
using System.IO;
using System.Linq;

namespace yukicore
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var tmp = Console.ReadLine()
                .Split(' ').Select(x => int.Parse(x))
                .ToArray();
            var n = (1.0*tmp[0] / (tmp[1] * 2));
            if (n <= 1) n = 0;
            else if (n % 1 == 0) n--;

            
            Console.WriteLine(
           (int)n*tmp[1]
            );
        }

   
    }
}