You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 4-amino-6-[1-(8-methyl-3-pyridin-2-ylindolizin-2-yl)ethylamino]pyrimidine-5-carbonitrile by IUPAC nomenclature, provide its molecular formula.
user
C21H19N7
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