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

class Program
{
    static string InputPattern = "Input4";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("90 20");
            //40
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("100 50");
            //0
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("1111 11");
            //550
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();
        int[] wkArr = InputList[0].Split(' ').Select(X => int.Parse(X)).ToArray();
        int L = wkArr[0];
        int K = wkArr[1];

        int EatLen = 0;
        while (L > 0) {
            if (L - 2 * K <= 0) break;
            L -= 2 * K;
            EatLen += K;
        }
        Console.WriteLine(EatLen);
    }
}