# Why DLDAG is a challenge problem?

I am having hard time understand how DLDAG is a challenge problem. ~~If ~~
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. If S is the minimum number of steps required (and we have to find S ~~required, ~~to be AC) then isn't C ~~uniquely determined?~~is n - S?