You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1R,1aR,6S,6aR)-1a,6a-dimethyl-1-phenyl-6-prop-1-en-2-yl-1,6-dihydrocyclopropa[a]indene The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H22
assistant