You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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For the molecule named 6a,9a-dichloro-2-hydroxy-6-(4-hydroxyphenyl)-8-methyl-3a,4,6,10,10a,10b-hexahydroisoindolo[5,6-e]isoindole-1,3,7,9-tetrone by IUPAC nomenclature, provide its molecular formula.
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C21H18Cl2N2O6
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