What word should I use to roughly describe the parts of the beginning of an educational material?
Example1. There are 10 videos of a math series lessons. And I want to express roughly that from the beginning the videos are good, and the rest are bad.
Example2. The first several chapters of a book are good and the rest are bad.
How to express that in a natural way?
(It doesn't have to match this pattern)
"The ______ of the videos are good, but the ______ are bad".
Mainly I want to express:
The (beginning parts) of the videos are good, but (the parts after that) are bad. (This sounds horrible)