You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
1-[4-[3-[3,4-dihydroxy-4-(hydroxymethyl)oxolan-2-yl]oxy-4,5-dihydroxy-6-(hydroxymethyl)oxan-2-yl]oxy-2-hydroxy-6-methoxyphenyl]ethanone is the IUPAC name of a molecule. Please give its molecular formula.
user
C20H28O13
assistant