using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace algo {
    class Program
    {
        static void Main(string[] args)
        {
            string[] N = Console.ReadLine().Split(' ');
            ulong L = ulong.Parse(N[0]);
            ulong K = ulong.Parse(N[1]);
            ulong ans = 0;

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