I would like to find a formula that would make the number portion of an alphanumeric string minus one. Is it possible to do it in one formula or must it be done with two separate formulas?

Example:
Original string: B300,
Formula makes it: B299,

Thank you.

Unable to make it work in one formula, can only make it with 2 separate formulas:

=MID(B2,2,3)-1
=”P”&G2&”,”

Note: sometimes it is P that precedes the number and sometimes it is R.