You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[4-[4-[(5-chlorothiophen-2-yl)methyl]piperazin-1-yl]-4-oxobutyl]-3H-quinazolin-4-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H23ClN4O2S
assistant