I was beaten by newtesrmakers having gauss solution
in __int64- overflow.
Rewrited the same program in my long arithm.
300- digits- MLE
200-digits-not  enough.
270- digits- AC.
Some new tests and the problem will become unsolvable.

You can use gauss over prime modular ring instead of long arithmetics
Just select lucky module =)

Thanks Godz my ID at Valladolid is prime! :D

Great idea) I used ortogonalization over prime module) Thanks.