You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
ethyl 3-[(3S)-3-(3-nitrophenyl)-5-phenyl-3,4-dihydropyrazol-2-yl]but-2-enoate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H21N3O4
assistant