Ok, so you need, if, for example, 5 is inputed, to show you 7, and if 7 is inputed to shouw you 11, and so on and so forth.
Well, we'll need a proof that you tried to solve your problem, so that we know where you're stuck. Post what you've done so far:).
For Each ctrl As Control In Me.Controls("pnlMainPanel").Controls
If ctrl.GetType Is GetType(System.Windows.Forms.Panel) Then
For Each subCtrl As Control In ctrl.Controls
If subCtrl.GetType Is GetType(System.Windows.Forms.TextBox) Then
If subCtrl.GetType Is ...