You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
N-[(1Z)-1-(2-fluoro-4-methoxyphenyl)buta-1,3-dienyl]-N-methyl-3,6-dihydro-2H-azepin-2-amine is the IUPAC name of a molecule. Please give its molecular formula.
user
C18H21FN2O
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