You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named 4-[(3aR,7aS)-5-methyl-7a-(3-methyl-1,2,4-oxadiazol-5-yl)-1,3,3a,4,6,7-hexahydropyrrolo[3,4-c]pyridin-2-yl]-2-chlorobenzonitrile by IUPAC nomenclature, provide its molecular formula.
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C18H20ClN5O
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