You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (8S)-3-benzyl-8-(3-fluorophenyl)-6-oxo-3,4,7,8-tetrahydro-2H-pyrido[2,1-b][1,3,5]thiadiazin-3-ium-9-carbonitrile by IUPAC nomenclature, provide its molecular formula.
user
C21H19FN3OS+
assistant