You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
methyl (1S,3aS,8aR)-5-methylidene-2'-oxospiro[3,4,6,7,8,8a-hexahydro-2H-azulene-1,4'-oxolane]-3a-carboxylate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C16H22O4
assistant