You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
4-[(1R,2R,9S,10R,11S,12R,16R)-6,13,15-trioxo-9-thiophen-2-yl-3,7-dithia-5,14-diazapentacyclo[9.5.1.02,10.04,8.012,16]heptadec-4(8)-en-14-yl]butanoic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C21H20N2O5S3
assistant