You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (2-oxido-3-oxo-1H-isoindol-2-ium-2-yl) (7,7-dimethyl-2-oxo-1-bicyclo[2.2.1]heptanyl)methanesulfonate by IUPAC nomenclature, provide its molecular formula.
user
C18H21NO6S
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