Not bad. Learning and thinking, good reading comprehension, big customs clearance is called dabaiben, which is very difficult. It's the ceiling of elementary school olympiad, which is much more difficult than the common analogy. The title of Dabaiben is different from Gauss's instruction in terms of its arch style. 1-2 is enlightenment, and after level 3, it is called training, generally called small white book. Relatively speaking, it is not difficult. Small white is the foundation, and big white is the upgrade.