You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1'-benzyl-5-fluorospiro[2,4-dihydro-1lambda6,2,4-benzothiadiazine-3,4'-piperidine] 1,1-dioxide is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H20FN3O2S
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