Private Sub Worksheet_Change(ByVal Target As Range)
Dim Cel As Range
Application.EnableEvents = False
On Error Resume Next
For Each Cel In Intersect(Me.Columns(2), Target)
Cel.Offset(, -1).FormulaR1C1 = "=IF(RC[1]="""","""",ROW()-1)"
Next Cel
Application.EnableEvents = True
End Sub