You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(8R,9R,10R,13S,14R,17R)-3-methoxy-10,13-dimethylspiro[2,7,8,9,12,14,15,16-octahydro-1H-cyclopenta[a]phenanthrene-17,2'-oxirane]-11-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H28O3
assistant