Why DLDAG is a challenge problem?

I am having hard time understand how DLDAG is a challenge problem.
we have: “The score for each test file is 10⋅(C/S)2, where C is the number of steps in which you deleted exactly 2 vertices. The final score of your submission is equal to the sum of scores for each test file.” On the other hand we know that at each step we can delete at most 2 vertices. And question asks us to find S to be the minimum number of steps required (and we have to print S to get AC) then isn’t C is n - S?

Well , I also can’t understand what really C is ?? But yes what we can do is to reduce no. of steps to delete DAG.

edited my initial question to be more clear. My problem is that question asks us to find minimum S to get AC.