You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[4-(1,3-benzothiazol-2-yl)piperazin-1-yl]-4-(4-chlorophenyl)sulfanylbutan-1-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H22ClN3OS2
assistant