You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
methyl (3S,5R,7aR)-3-hydroxy-3a,7a-dimethyl-4-oxo-1,2,3,5,6,7-hexahydroindene-5-carboxylate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C13H20O4
assistant