You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named (5aS,7S,9aR)-9a-benzyl-7-(2-chloroethynyl)-4,5,5a,6,8,9-hexahydro-1H-benzo[g]indazol-7-ol by IUPAC nomenclature, provide its molecular formula.
user
C20H21ClN2O
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