Let $a_n$, $n \in \mathbb{N}$, be an A.P. with common difference $d$ and all non-zero terms. As $n$ approaches infinity, the sum $\dfrac{1}{a_1 a_2} + \dfrac{1}{a_2 a_3} + \cdots + \dfrac{1}{a_n a_{n+1}} + \cdots$ will approach