You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (2,6-difluorophenyl)-[4-(3,5-dimethylpyridin-2-yl)piperazin-1-yl]methanone by IUPAC nomenclature, provide its molecular formula.
user
C18H19F2N3O
assistant