You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
methyl [(3E)-3-ethylidene-7,7-dimethyl-2-oxo-1-bicyclo[2.2.1]heptanyl]methanesulfonate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C13H20O4S
assistant