You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1S,15R,18S,19R,20S)-19-(chloromethyl)-1,3,11,12,14,15,16,17,18,19,20,21-dodecahydroyohimban-18-ol is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H25ClN2O
assistant