You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
5,9-dimethyl-14-methylidene-15-oxotetracyclo[11.2.1.01,10.04,9]hexadec-13(16)-ene-5-carboxylic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H26O3
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