You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
3-ethyl-2-(pyridin-2-ylmethyl)-1,3,4,6,7,8,9,9a-octahydropyrido[1,2-a]pyrazine The above is the IUPAC name of a molecule. Write its molecular formula.
user
C16H25N3
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