直接二分结果，精度很高，1e-4可过，1e-3未试。

核心代码：
bool can(double D)
{
	double R = (sqr(L)/4 + sqr(D)) / (2*D);
	double sita = asin(L/2 / R) * 2;
	double Len = 2 * sita * R * 0.5;
	if (cmp(Len - Length)<=0)
		return true;
	return false;
}

while (High-Low >1e-4)
{
	double Mid = (Low + High)/2;
	if (can(Mid))
		Low = Mid;
	else
		High = Mid;
}