Private Sub Worksheet_Change(ByVal Target As Range)
    On Error Resume Next
    If Not Intersect(Target, Range("C13:C43")) Is Nothing And Target.Cells.Count = 1 Then
        Application.EnableEvents = False
        newVal = Target
        Application.Undo
        oldval = Target
        If Len(oldval) <> 0 And oldval <> newVal Then
            Target = Target & "-" & newVal
        Else
            Target = newVal
        End If
        If Len(newVal) = 0 Then Target.ClearContents
        Application.EnableEvents = True
    End If
End Sub