You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-[5-[(1-methylindol-3-yl)methylidene]-4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl]pentanoic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H18N2O3S2
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