I am just getting to learn c# and I am attempting to calculate different times and prices associated to a movie theater type of calculation but when I choose two different times it add the first time chosen and prices and multiplies it by the new time and amount instead of adding the two different times together. Some information to help fix my problem will be greatly appreciated.

Code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        double ctix;
        double atix;
        double stix;
        double price;
        double price1;
        double tax;
        double total1;
       

        ctix = Convert.ToDouble(TextBox2.Text);
        atix = Convert.ToDouble(TextBox3.Text);
        stix = Convert.ToDouble(TextBox4.Text);

        if (CheckBox1.Checked == true)
        {
            ctix = ctix * 5.00;
            atix = atix * 5.00;
            stix = stix * 5.00;
        };

        if (CheckBox2.Checked == true || CheckBox3.Checked == true)
        {
            ctix = ctix * 6.50;
            atix = atix * 6.50;
            stix = stix * 6.50;

        };

        if (CheckBox4.Checked == true || CheckBox5.Checked == true)
        {
            ctix = ctix * 6.50;
            atix = atix * 9.50;
            stix = stix * 7.00;
            
        };

        
        price = ctix + atix + stix;

        price1 = price * .835;

        tax = price - price1;

        total1 = price + tax;

        TextBox5.Text = price.ToString();
        TextBox6.Text = tax.ToString();
        TextBox7.Text = total1.ToString();








    }
}