what's the abbreviation for Puerto Rico?

Pharmacist: Is this a license? Me: Yes Pharmacist: (under his breath) Ugh what am i supposed to do with this? Pharmacist: Ugh its asking me for the ... what's the abbreviation for Puerto Rico? [Me in my head: really? REALLY !!!] Me: P. R. Pharmacist: oh ok lesson learned: I need to get my MN license on Friday