Consider a parabola $4y = x^2$ and point $B(0,1)$. Let $A_1\left(x_1, y_1\right), A_2\left(x_2, y_2\right), \ldots\ldots\ldots\ldots, A_n\left(x_n, y_n\right)$ are $n$ points on the parabola such that $x_r > 0$ and $\angle OBA_r = \frac{r\pi}{2n}$ $(r = 1, 2, \ldots\ldots\ldots, n)$ then $\pi\left(\lim_{n \to \infty} \frac{1}{n} \sum_{r=1}^{n} BA_r\right)$ is equal to ______.