Rihanna -- Judges Obama's Fallon Gig ... I'd Give Him an ___ (VIDEO)

Rihanna is quick to judge Prez Obama's performance of her song, it's just hard to hear what she says.

It's clear RiRi liked Obama's performance of "Work" on Jimmy Fallon's show, but when it comes to the real stuff -- judging the mic drop -- she seems to give a number. We just can't hear it.

Help us out..