Saisie obligatoire sous condition

Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False

If Range("A1") = "" And Target.Address <> "$A$1" Then
Application.Undo
Range("A1").Select
End If

If Range("B1") = "" And Target.Address <> "$A$1" And Target.Address <> "$B$1" Then
Application.Undo
Range("B1").Select
End If

Application.EnableEvents = True
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Target.Column = 2 Then
  If IsEmpty(Target.Offset(0, -1).Value) Then
    Application.Undo
    Target.Offset(0, -1).Select
  End If
End If
Application.EnableEvents = True
End Sub