You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Translate the chemical IUPAC name (4S,5R)-1-ethyl-5-pyridin-3-yl-4-[(7H-pyrrolo[2,3-d]pyrimidin-4-ylamino)methyl]pyrrolidin-2-one into its molecular formula.
user
C18H20N6O
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