You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[(2S)-butan-2-yl]sulfanyl-3-(4-fluorophenyl)-5H-pyrimido[5,4-b]indol-4-one The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H18FN3OS
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