You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name (3'aR,8'bS)-5-bromo-2'-butan-2-ylspiro[1H-indole-3,4'-3a,6,7,8,8a,8b-hexahydropyrrolo[3,4-a]pyrrolizine]-1',2,3'-trione into its molecular formula.
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C20H22BrN3O3
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