You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(9'E,11'E)-16'-chloro-17',19'-dihydroxy-4'-methylspiro[1,3-dithiane-2,13'-7-oxa-3-azatricyclo[13.4.0.06,8]nonadeca-1(15),9,11,16,18-pentaene]-2'-one is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H24ClNO4S2
assistant