Calculation Part in checkcbox check

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 WindowsFormsApplication1
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }
        private void Display()
        {
            int t = 0;
            if (Chk500.Checked)
                t += 500;
            if (Chk1000.Checked)
                t += 1000;
            if (Chk1500.Checked)
                t += 1500;
            TxtTotal.Text = t.ToString();
        }
        private void Chk500_CheckedChanged(object sender, EventArgs e)
        {
            Display();
        }

        private void Chk1000_CheckedChanged(object sender, EventArgs e)
        {
            Display();
        }

        private void Chk1500_CheckedChanged(object sender, EventArgs e)
        {
            Display();
        }
      
    }
}
Note :(+= )means It will add Previous Value and Current Value.