You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
ethyl 2-[(3R,3aS,5aS,8aS,8bS)-4-methyl-3-(2-oxopropyl)-3,3a,5a,6,7,8,8a,8b-octahydro-as-indacen-2-yl]acetate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H28O3
assistant