You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(5R)-1-(3-chlorophenyl)-4-[2-(1-ethylpiperidin-4-yl)acetyl]-5-methylpiperazin-2-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H28ClN3O2
assistant