Private vData
 
Private Sub Worksheet_SelectionChange(ByVal Target As Excel.Range)
    If Target.Count > 1 Then Exit Sub
    If Not Intersect(Target, [A1:A100]) Is Nothing Then
       If IsNumeric(Target) Then vData = Target
    End If
End Sub
 
Private Sub Worksheet_Change(ByVal Target As Excel.Range)
    If Not Intersect(Target, [A1:A100]) Is Nothing Then
       Application.EnableEvents = False
       If Target.Count = 1 And IsNumeric(Target(1)) Then
          Target = Target + vData
       Else
          Application.Undo
       End If
       Application.EnableEvents = True
    End If
End Sub