You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named [(9S,13S)-10-(cyclopropylmethyl)-4,13-dihydroxy-13-(3-oxobutyl)-10-azatricyclo[7.3.1.02,7]trideca-2(7),3,5-trien-3-yl]oxidanium by IUPAC nomenclature, provide its molecular formula.
user
C20H28NO4+
assistant