You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Convert the IUPAC name of a molecule N-[1-(4-methyl-3-nitrophenyl)ethyl]-2,3,3a,4,5,6,7,7a-octahydro-1H-indole-2-carboxamide into molecular formula.
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C18H25N3O3
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