You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name (1S,3R,3aR,6aS)-5'-bromo-5-butyl-1-methylspiro[1,2,3a,6a-tetrahydropyrrolo[3,4-c]pyrrol-2-ium-3,3'-1H-indole]-2',4,6-trione into its molecular formula.
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C18H21BrN3O3+
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