You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3,9-diazabicyclo[4.2.1]nonan-3-yl-[1-(3-methyl-4-nitrobenzoyl)pyrrolidin-2-yl]methanone;hydrochloride is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H27ClN4O4
assistant