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

public class Program
{
    public void Proc(){
        Reader.IsDebug = false;
        int[] inpt = Reader.GetInt();
        int pockyLength = inpt[0];
        int eat = inpt[1];
        
        int yuEated = 0;
        while (pockyLength > 0)
        {
            if(pockyLength > eat * 2) {
                yuEated += eat;
                pockyLength -= (eat*2);
            } else
            {
                break;
            }
        }
        Console.WriteLine(yuEated);
    }

    public static void Main(string[] args)
    {
        Program prg = new Program();
        prg.Proc();
    }
}

class Reader
{
    public static bool IsDebug = true;
    
    private static System.IO.StringReader sr;
    
    public static string ReadLine() {
        if(IsDebug) {
            if(sr == null) {
                sr = new System.IO.StringReader(initStr.Trim());
            }
            return sr.ReadLine();
        } else {
            return Console.ReadLine();
        }
    }
    
    public static int[] GetInt(char delimiter = ' ') {
        string[] inpt = ReadLine().Split(delimiter);
        int[] ret = new int[inpt.Length];
        for(int i=0; i<inpt.Length; i++) {
            ret[i] = int.Parse(inpt[i]);
        }
        return ret;
    }
    
    private static string initStr = @"


1111 11


    ";
}