You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Convert the IUPAC name of a molecule N-[2-[(1R)-1-(3,4-dichlorophenyl)-3,4-dihydro-1H-pyrrolo[1,2-a]pyrazin-2-yl]-2-oxoethyl]-N-ethylmethanesulfonamide into molecular formula.
user
C18H21Cl2N3O3S
assistant