You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
(1R,4aS,10aR)-7-ethyl-1,4a-dimethyl-2,3,4,9,10,10a-hexahydrophenanthrene-1-carboxylic acid is the IUPAC name of a molecule. Please give its molecular formula.
user
C19H26O2
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