You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[(1S,3aS,6R,7aS)-6-hydroxy-4-[2-[(5R)-5-hydroxy-2-methylidenecyclohexylidene]ethylidene]-7a-methyl-2,3,3a,5,6,7-hexahydro-1H-inden-1-yl]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H30O3
assistant