On addition of row in Subform, I want to have an auto increment values

On addition of row in Subform, I want to have an auto increment values

Hi Everyone, I have a subform and I added a single line field inside the subform.
Now I want to have an auto increment number based on the number of rows inside the subform. start in 1.
TRIGGER : ON ADDITION OF ROW.
(i.e)
1
2
3
and so on.


Regards,