You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
2-piperazin-1-yl-5-[2-(2-pyridin-2-ylethenyl)pyridin-4-yl]-1H-pyrrole-3-carbonitrile The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H20N6
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