You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(9-methyl-4-oxopyrido[1,2-a]pyrimidin-2-yl)methyl 1-ethylpiperidine-2-carboxylate is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H23N3O3
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