You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 4-(5-methylthiophen-2-yl)-3a,4,5,9b-tetrahydro-3H-cyclopenta[c]quinoline-6-carboxamide by IUPAC nomenclature, provide its molecular formula.
user
C18H18N2OS
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