You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-amino-5-(4-methylsulfanylphenyl)-3,7,8,9-tetrahydropyrimido[4,5-b]quinoline-4,6-dione The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H16N4O2S
assistant