Hi

I am solving a problem that is conceptually simple.

Given an integer x, such that 1 ≤ x ≤ 10¹⁷, check if v = 5x² + 2x + 1 is a perfect square. I have solved this using Big Integers and long doubles in C++ (with a binary search to find the root) and used PyPy to get it accepted in Python but I did not receive satisfaction in solving it this way.

Also, __int128 do not work on SPOJ for some reason :/

I think Big Integer may be an overkill for such a problem and the uncertainty with floating point precision is never re-assuring. Hence, my question is, is there any way to solve this problem without Big Integers or long doubles in C++?

Thanks