You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
[2-(3-chloro-4-methoxyphenyl)-2-oxoethyl] 2-[(1R,2R,3S)-3-methyl-2-(nitromethyl)-5-oxocyclopentyl]acetate is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20ClNO7
assistant