You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
[2-[4-(4-nitrophenyl)piperazin-1-yl]-2-oxoethyl] 2,3-dihydro-1-benzothiophene-2-carboxylate is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H21N3O5S
assistant