dBs

I'm feeling cognitively-declined today, probably as a consequence of my
vast age and general ignorance of matters mathematical and everything else
in fact, with the sole exception of "fatuous conspiracy theories." Can
some kind soul assist?
If my RF power meter is reading -13dbm when there's a 20dB attenuator in
line, what is the true power level, please?
I've got an exhaustive App Note from Rhode & Schwartz which claims to
cover everything about decibels, but, er, doesn't.

CD.