Find the value of $\tan ^{-1}\left(\frac{a_{1} x-y}{a_{1} y+x}\right)+\tan ^{-1}\left(\frac{a_{2}-a_{1}}{1+a_{1} a_{2}}\right)$
$+\tan ^{-1}\left(\frac{a_{3}-a_{2}}{1+a_{3} a_{2}}\right)+\ldots .+\tan ^{-1}\left(\frac{a_{n}-a_{n-1}}{1+a_{n} a_{n-1}}\right)$
$+\tan ^{-1}\left(\frac{1}{a_{n}}\right)$, where $x, y, a_{1}, a_{2}, \ldots a_{n} \in R^{+}$