You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(3R,8aS)-2-[(E)-3-(4-methoxyphenyl)prop-2-enyl]-3-methyl-3,4,6,7,8,8a-hexahydro-1H-pyrrolo[1,2-a]pyrazine is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H26N2O
assistant