You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Convert the IUPAC name of a molecule (4R,4aR,7aR,12bR)-5-chloro-9-methoxy-3-methyl-1,2,4,4a,7a,11a,12a,13-octahydro-4,12-methanobenzofuro[3,2-e]isoquinolin-7-one into molecular formula.
user
C18H20ClNO3
assistant