You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[4-(2,3,3a,4,5,6,7,7a-octahydroindol-1-yl)-3-bromophenyl]butan-2-amine is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H27BrN2
assistant