You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
5-benzyl-2-(4-methylpiperidine-1-carbonyl)-7,8-dihydro-6H-pyrazolo[1,5-a][1,4]diazepin-4-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H26N4O2
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