You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4-chloro-3-methylphenyl)-[4-(4-chloro-3-methylphenyl)-4-(hydroxymethyl)piperidin-3-yl]methanone The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H23Cl2NO2
assistant