why is solution with long double not working, long double must have 18 digits according to ISO c++, which must be enough to solve this problem

i've calculated everything using g++ and long double, and formed a constant and got AC, which means that system compiler is bad..

The solution for the hardest case is the number "138879579704209680000" which has 21 decimal digits.
You are just a lucky guy :) Maybe, because of its terminating zeroes, who knows.