You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-(5-methylthiophen-2-yl)sulfonyl-1-phenyl-3,4-dihydro-1H-pyrrolo[1,2-a]pyrazine The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H18N2O2S2
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