You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1S)-1-[(1S,3aR,7aS)-4-(bromomethylidene)-7a-methyl-2,3,3a,5,6,7-hexahydro-1H-inden-1-yl]ethanol is the IUPAC name of a molecule. Please give its molecular formula.
user
C13H21BrO
assistant