Spoj Venom - Binary Search

There is this question on Spoj called VENOM . There are 2 approaches to solve this question .

One approach is by quadratic roots . takes o(1).

Other is by binary search .

Here, take a look at this tutorial on how to go about by binary search.

The problem is that i cannot find the upper and lower bounds between which binary search is to be found.
Please tell me how to do it ?

//