Let \( a_1 = b_1 = 1, \ a_n = a_{n-1} + 2 \ \& \ b_n = a_n + b_{n-1} \) for every natural number \( n \ge 2 \). Then \( \displaystyle\sum_{n=1}^{15} a_n \cdot b_n \) is equal to ________.