You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[(1S)-3-(7-fluoro-1,2,3,4-tetrahydrocyclopenta[b]indol-8-yl)cyclohex-3-en-1-yl]but-2-ynamide is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H21FN2O
assistant