You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3E)-3-[(4-methyl-5-oxo-2H-furan-2-yl)oxymethylidene]-3a,4,5,9b-tetrahydrobenzo[g][1]benzofuran-2-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H16O5
assistant