You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
[4-[(5-chlorothiophen-2-yl)methyl]piperazin-1-yl]-(5-methyl-1-phenylpyrazol-3-yl)methanone The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H21ClN4OS
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