1. I spent nearly a half of hour to clear that "a dangerous stuff" does not mean "student may be poisoned", but "he will be poisoned exactly because of the stuff". Why not use "a harmful stuff" everywhere?
2. And these words: "The food is cooked from M different food stuffs. There are N different food stuffs in the menu but not all of them are at the distribution...".
Why not write "The distribution contains M different food stuffs from the menu. The menu consists of N different stuffs."?
3. So, I've understood that the distribution does not contains the stuffs from the first block. But it took much time because of bad description.

Agree... simple problem, awful problem statement.