How stupid I am.
At first I think I should use "bignum",so I kept coding and coding for about two hours.
But I found that it's so complex.
So I stopped and start reading the board.
Oh my dear,I realized that the "double"'s range is about from -3e+302 to +3e+302.
So everybody who see this don't try to use "bignum",double is enough.

Double can of course get AC... But the tests seem to be really weak (the so-called 2 or 3 coords algorithms are actually wrong).
But the correct algorithm is not hard to write. Why not do it in this way?