\int_{\frac{-1}{2}}^0 \sqrt{\frac{1}{1-4 x^2}} \, dx\leq \ell \leq \lim_{e\to 0} \int_{-\left(\frac{1 }{2}+e\right)}^0 \sqrt{\frac{(1+2 e)^2}{1+4 e+4 e^2-4 x^2}} \, dx