c = (l + r) / 2;
			al = (-4 * c) / L;
			a = (-4 * c) / (L * L);
			lo = logl(al + sqrtl((double)(1 + al*al)));
			L2 = (al*sqrtl((double)(1 + al*al)) + lo) / (2 * a);
			L1 = (1 + n*C)*L;
			if (judge(L2, L1))	r = c;
			else l = c,ans=c;