Convert the IUPAC name of a molecule (1R,2S,3S,4R,5'R,7S,8R,9S,12S,13S,15R,16R,18S)-5',7,9,13-tetramethylspiro[5-oxapentacyclo[10.8.0.02,9.04,8.013,18]icosane-6,2'-oxane]-3,15,16-triol into molecular formula.