You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1R,4Z,7R,11S,12S,14S,17R)-4-ethylidene-7,12-dihydroxy-7-methyl-6-methylidene-14-oxido-2,9-dioxa-14-azoniatricyclo[9.5.1.014,17]heptadecane-3,8-dione is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H25NO7
assistant