You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 5,10,16,21-tetrazaheptacyclo[13.9.1.02,10.04,9.011,25.016,24.017,22]pentacosa-4(9),5,7,11(25),12,14,17(22),18-octaene by IUPAC nomenclature, provide its molecular formula.
user
C21H18N4
assistant