You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(8R)-3-benzyl-8-(4-fluorophenyl)-6-oxo-2,4,7,8-tetrahydropyrido[2,1-b][1,3,5]thiadiazine-9-carbonitrile The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H18FN3OS
assistant