Get Value from TextBox and add to listbox in vb.net 2008

Public Class exam
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim n As String = txt_theory.Text 'get i/p from user for no.of times
Dim temp As String = txt_theoryent.Text 'get data from user
Dim i As Integer
For i = 1 To n
lst_theory.Items.Add(CType(temp, String))
Next i
End Sub

This is my code. For loop was not working. pls correct it.
I am using 2 textbox, 1 listbox, 1 button.
for eg:
Textbox1 = Enter the no.of subjects
Textbox2 = Enter 1 subject & Add to listbox by press button.
repeat this subject entry upto textbox1 value..