You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Translate the chemical IUPAC name (4S)-7'-methyl-1'-(2-methylpropyl)spiro[5,7-dihydro-1H-pyrazolo[3,4-b]pyridine-4,3'-indole]-2',6-dione into its molecular formula.
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C18H20N4O2
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