Print No's In listbox

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Forms;

namespace WindowsFormsApplication1

{

    public partial class printno : Form

    {

        public printno()

        {

            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)

        {

            for (int i = 1; i <= 20; i++)

            {

                listBox1.Items.Add(i);

            }

        }      

    }

}