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

namespace yukicoderTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string input = Console.ReadLine();
            string[] LK = input.Split(' ');
            int L = int.Parse(LK[0]);
            int K = int.Parse(LK[1]);
            int cm = 0;
            while(L>K*2)
            {
                L = L - K * 2;
                cm = cm + K;
            }
            Console.WriteLine(cm);
        }
    }
}