You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
Please write the molecular formula of the molecule (4-nitrophenyl)methyl (5R)-6-[acetyloxy(pyrazin-2-yl)methyl]-7-oxo-4-thia-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylate .
user
C20H16N4O7S
assistant