Hotel Room Calculation Help

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace RoomLogic
{
    public partial class Form1 : Form
    {
        private const decimal StandardDoubleRate = 189.99M; //price for a standard double room
        private const decimal DeLuxeDoubleRate = 199.99M; //price for a DeLuxe double room
        private const decimal LuxuryDoubleRate = 229.99M; //price for a Luxury Double room
        private const decimal LuxuryKingRate = 339.99M; //price for a Luxury King rooom

        //sales tax for the state
        private const decimal SalesTaxRate = 0.0825M;

        //current rate
        private decimal StandardRate, DeLuxeRate, DoubleRate, KingRate, TotalRoomsRate;

        //date time variables to get the check in and check out dates
        private DateTime DateIn;
        private DateTime DateOut;
        
        //variables to hold the number of children and adults in each room
        private decimal AdultsTotal;
        private decimal ChildrenTotal;
        private decimal TotalRooms;
        private decimal ExtraAdultCost;
        private decimal TotalCost;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.Text = this.Text + "   " + DateTime.Now.ToShortDateString() + "   " + DateTime.Now.ToShortDateString();

            dtpIn.Value = DateTime.Today; //shows the check in date time picker as today's date

            dtpOut.Value = dtpIn.Value.AddDays(1); //shows the check out date time picker as today's date.
        }//end form load event

        //check changed event
        private void chkStandard_CheckedChanged(object sender, EventArgs e)
        {
            StandardRate = StandardDoubleRate * numStandard.Value;
        }

        private void chkDeLuxe_CheckedChanged(object sender, EventArgs e)
        {
            DeLuxeRate = DeLuxeDoubleRate * numDeLuxe.Value;
        }

        private void chkDouble_CheckedChanged(object sender, EventArgs e)
        {
            DoubleRate = LuxuryDoubleRate * numDouble.Value;
        }

        private void chkKing_CheckedChanged(object sender, EventArgs e)
        {
            KingRate = LuxuryKingRate * numKing.Value;
        }

        # region Calculations

        public void btnCalc_Click(object sender, EventArgs e)
        {
            //clears list for neatness each time the room rate is calculated
            lstOutput.Items.Clear();

            long Days;
            decimal CostDays;

            /* All calculations for room cost will take place inside this button.
             * Steps:
             * 1.  Calc the number of days the people will be staying in the room(s).
             * 2.  Calcuate the cost of the rooms(s) based on that.
             * 
             * calculate the number of elasped days and return a long integer to calculate number of days from start to finish.
             */

            DateIn = dtpIn.Value;
            DateOut = dtpOut.Value;

            Days = DateOut.Subtract(DateIn).Days;
            MessageBox.Show("Total days = " + Days.ToString());

            MessageBox.Show("Standard Total: " + StandardRate);
            MessageBox.Show("DeLuxe Total: " + DeLuxeRate);
            MessageBox.Show("King Total: " + KingRate);

            //calculate the Room cost based on days needed
            TotalRoomsRate = StandardRate + DeLuxeRate + DoubleRate + KingRate;

            CostDays = TotalRoomsRate * Days;

            AdultsTotal = this.numAdults.Value;
            ChildrenTotal = this.numChildren.Value;

            decimal TotalPeople = AdultsTotal + ChildrenTotal;
            TotalRooms = numStandard.Value + numDeLuxe.Value + numDouble.Value + numKing.Value;
            //MessageBox.Show("Total Rooms = " + TotalRooms.ToString());  Working

            //if statment to decide if enough rooms are purchased for the party.
            if (AdultsTotal > 4 & TotalRooms < 2)
            {
                MessageBox.Show("Must reserve/purchase additional rooms to accomodate party amount.");
            }
            else if (ChildrenTotal > 4 & TotalRooms < 2)
            {
                MessageBox.Show("Must reserve/purchase additional rooms to accomodate party amount.");
            }

            if (AdultsTotal > 2 & AdultsTotal < 5 & TotalRooms < 2)
            {
                ExtraAdultCost = (AdultsTotal - 2) * 10;
            }
            else if (AdultsTotal > 4 & AdultsTotal < 9 & TotalRooms < 3)
            {
                ExtraAdultCost = (AdultsTotal - 4) * 10;
            }
            else if (AdultsTotal > 6 & AdultsTotal < 13 & TotalRooms < 4)
            {
                ExtraAdultCost = (AdultsTotal - 6) * 10;
            }

            TotalCost = ((CostDays + ExtraAdultCost) * SalesTaxRate) + CostDays + ExtraAdultCost;
            lstOutput.Items.Add("The total room cost for your stay is: " + CostDays.ToString("C"));
            lstOutput.Items.Add("The cost for extra adults staying is: " + ExtraAdultCost.ToString("C"));
            lstOutput.Items.Add("The total cost of your stay is: " + TotalCost.ToString("C"));
            
        #endregion

        }
    }
}