You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[4-(4-chloro-2-nitrophenyl)piperazin-1-yl]-2-[methyl-[(4-methylphenyl)methyl]amino]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H25ClN4O3
assistant