You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 1-[(3aS,7R,7aR)-7-ethyl-7-hydroxy-3,3a,4,5,6,7a-hexahydro-1H-isoindol-2-yl]-3-(2,4-dimethylphenyl)propan-1-one by IUPAC nomenclature, provide its molecular formula.
user
C21H31NO2
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