You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named (4R,10S,13S)-4-ethyl-10-(hydroxymethyl)-3-imino-13-methyl-1,2,4,5,7,8,9,11,12,14,15,16-dodecahydrocyclopenta[a]phenanthrene-6,17-dione by IUPAC nomenclature, provide its molecular formula.
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C21H31NO3
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