The problem states that the root should be in the range of (0, n - 1), which looks like saying 0 < x < n - 1. However apparently x can be equal to n - 1! I spent several hours stuck on WA 2 just because of this. Please change it to [0, n - 1] or whatever is precise.

Could you please next test case in statement?
9 7 (ans is same as for 2 7, for people to understand that a can be greater than n)
Because it's not precise and personally I spent 2-3 hours just to add (%n) stuff.