Inverse trig functions!

$\cot^{-1}(3) + \cot^{-1} (7) + \cot^{-1} (13) + \cot^{-1}(21) + \ldots = \ ?$

Clarification : $3,7,13,21,\ldots$ is an increasing progression of integers for which the difference between the $n$ -th and $(n-1)$ -th term is equals to $2n$ .