using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

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

            var eated = 0;
            var result = 0;

            while(eated + inputs[1] * 2 < inputs[0])
            {
                eated += inputs[1] * 2;
                result += inputs[1];
            }

            Console.WriteLine(result);
        }
    }
}