You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
ethyl (3R,3aR,9bS)-2-oxo-1-phenyl-3,3a,4,9b-tetrahydrochromeno[4,3-b]pyrrole-3-carboxylate The above is the IUPAC name of a molecule. Write its molecular formula.
user
C20H19NO4
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