How do I stop a Macro after "Cancel" is chosen in an input box?

I have the following code:

Msg = ("Enter an account number to highlight.")
   UserEntry1 = InputBox(Msg)
   Ans = MsgBox("Would you like to highlight an additional account?", vbYesNo)
   Select Case Ans
       Case vbNo
           Worksheets("R1").Select
           Counter = 0
           EndCounter = 20
           Do Until Counter = EndCounter
               Range("F6:F28").Select
               Selection.FormatConditions.Delete
...

If I press cancel in the first Msg I can't stop the entire rutine. How do I do that?

/Møller