You are a chemist. Please follow the instructions to convert the structure to the corresponding name.
system
For the molecule named 1-[(5S,7S)-5-(4-chlorophenyl)-7-(2-methoxyphenyl)-6,7-dihydro-5H-[1,2,4]triazolo[1,5-a]pyrimidin-4-yl]ethanone by IUPAC nomenclature, provide its molecular formula.
user
C20H19ClN4O2
assistant