You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(2-chloropyridin-4-yl)-[4-[[5-(2-fluorophenyl)thiophen-2-yl]methyl]piperazin-1-yl]methanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H19ClFN3OS
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