You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (3aR,4R,6aS)-2'-methyl-3'-(2-methylpropyl)-2-(pyridin-3-ylmethyl)spiro[1,3,3a,5,6,6a-hexahydrocyclopenta[c]pyrrole-4,5'-imidazole]-4'-one by IUPAC nomenclature, provide its molecular formula.
user
C20H28N4O
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