You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
sodium;(5R,6S)-6-[(1R)-1-hydroxyethyl]-7-oxo-3-[4-(pyridine-2-carbonylamino)phenyl]-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H18N3NaO5
assistant