You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(4aR,7aS)-4-hydroxy-2-oxo-N-(4-phenoxyphenyl)-1,4a,5,6,7,7a-hexahydrocyclopenta[b]pyridine-3-carboxamide The above is the IUPAC name of a molecule. Write its molecular formula.
user
C21H20N2O4
assistant