You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 2-[(2R,4aR,8aS)-4a-methyl-8-methylidene-1,2,3,4,5,6,7,8a-octahydronaphthalen-2-yl]prop-2-enyl (Z)-2-methylbut-2-enoate by IUPAC nomenclature, provide its molecular formula.
user
C20H30O2
assistant