Let $a_1, a_2, \\dots, a_{20}$ be an arithmetic progression. If the sum of the reciprocal adjacent products is: $\\sum_{i=1}^{19} \\frac{1}{a_i a_{i+1}} = \\frac{19}{36}$ and the first term is $a_1 = 3$, find the last term $a_{20}$.