Numerotation lignes dans excel

Sub NumPlage()
    Dim num As Long
    Dim z As String
    Dim c As Range
    If Selection.Columns.Count <> 1 Then
        MsgBox ("Sélectionnez une seule colonne")
        Exit Sub
    End If
    If Selection.Row = 1 Then
        num = 1
    Else
        num = ActiveCell.Offset(-1, 0).Value + 1
    End If
    For Each c In Selection
        If c.MergeCells Then
            If c.MergeArea.Address <> z Then
                z = c.MergeArea.Address
                c.MergeArea.Cells(1, 1).Value = num
                num = num + 1
            End If
        Else
            c.Value = num
        num = num + 1
        End If
    Next c
End Sub