You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-benzyl-1'-(furan-2-carbonyl)spiro[6,8-dihydropyrrolo[2,1-c][1,2,4]triazine-7,3'-pyrrolidine]-3,4-dione is the IUPAC name of a molecule. Please give its molecular formula.
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C21H20N4O4
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