$I =\int_{ -\infty}^{\infty}\frac{dx}{1+x^2}$ = $[\tan^{-1} x]_{-\infty}^{\infty}$
$\quad = \tan^{-1}(\infty) - \tan ^{-1} (-\infty) = \tan^{-1} \left(\tan \frac{\pi}{2}\right) – \tan^{-1} \left(\tan \left(\frac{-\pi}{2}\right)\right) = \frac{\pi}{2} + \frac{\pi}{2} = \pi.$