Telescoping & Recursion · Medium problem

A sequence is defined by $a_1 = 1$ and $a_{n+1} = \dfrac{a_n}{1 + 3a_n}$ for $n \ge 1$. Find the value of $\displaystyle\sum_{n=1}^{33} a_n a_{n+1}$.

Express your answer as a common fraction.