Web Forms :: Prevent Label Update On Postback

I am checking to see on the form if they answer No to question one then go to comments. On the client side I added autopostback="true".   So when the page reloads the random number in the label is updating to a new number.  How can I avoid the random number not to be updated when the page post pack to itself?

Protected Sub form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles form1.Load
Dtetxt.Text = Date.Now
F()
End Sub
Private Sub F()
Dim _r As New Random()
Dim n As Integer = _r.[Next](5)
' Can return 0, 1, 2, 3, or 4
Label2.Text = n.ToString
End Sub