Need to sequentially number dataset in groups

I have data that I need to number sequentially, but in groups so that Column A will eventually look something like the following: 1,1,1,1,2,2,2,2,2,2,2,2,3,3,4,4,4,5,5,5, etc... dependent upon text in Column B.

Specifically, if Column B has the text "PRO," that indicates the beginning of a new group of rows and needs to have a unique ID # inserted into Column A. These ID numbers will range from 1-500 and run sequentially. Unfortunately, there is no consistency to how frequently a "PRO" entry appears and in total there are over 15,000 rows.

I have been searching for an answer for a while now and have tried filtering, IF, SUBTOTAL, and ROWS functions to no avail. I am not getting any closer to the solution just looking for the answer myself.

I attached a sample spreadsheet. Thanks for any input!

Awesome; that worked perfectly! Can't believe how simple it was. Thank you!