You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1,3,4,6,7,8,9,9a-octahydropyrido[1,2-a]pyrazin-2-yl-[5-(2-fluorophenyl)-1,2-oxazol-3-yl]methanone The above is the IUPAC name of a molecule. Write its molecular formula.
user
C18H20FN3O2
assistant