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

namespace lecture
{
    class Program
    {
     
        static void Main(string[] args){
            string[] line = Console.ReadLine().Split(' ');
            int L = int.Parse(line[0]);
            int K = int.Parse(line[1]);

            int count = 0;

            while ( L > 2 * K )
            {
                L -= 2 * K;
                count += K;
            }
            Console.WriteLine(count);
        }
    }
}