I need help with AutoFill VBA

Hello Everybody,

Thank you all you support.

My Problem I have this below code. I need this below code to modify as everytime I enter text in
column B the marco automatically need to fill serial number in column A. I mean

A B
1 venkat
2 vani
3 raju
ramu
roudy

Sub marco()
    If ActiveCell > 0 Then
        ActiveCell.Offset(0, -1).Value _
            = ActiveCell.Offset(-2, -1) + 2
            
    End If
    
End Sub

thank you for your help,
venkat