You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
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Under IUPAC nomenclature, a molecule named 2',4',5',7'-tetrabromo-4,5,6,7-tetrachloro-3-oxospiro[2H-indene-1,9'-xanthene]-3',6'-diolate is given. What is its corresponding molecular formula?
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C21H4Br4Cl4O4-2
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