You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (E)-2-[2-[(1S,2S,4aS,8aS)-1,2,4a,5-tetramethyl-2,3,4,7,8,8a-hexahydronaphthalen-1-yl]ethyl]but-2-enedioic acid by IUPAC nomenclature, provide its molecular formula.
user
C20H30O4
assistant