Let a_1= b_1=1, a_n = a_n -1 +2 and b_n = a_n + b_n -1 for every natural number n ≥ 2. Then Σ_n=1^15 a_n · b_n is equal to \ \ \ \.

Let $\displaystyle \mathrm{a}_1=\mathrm{b}_1=1, \mathrm{a}_{\mathrm{n}}=\mathrm{a}_{\mathrm{n}-1}+2$ and $\displaystyle \mathrm{b}_{\mathrm{n}}=\mathrm{a}_{\mathrm{n}}+\mathrm{b}_{\mathrm{n}-1}$ for every natural number $\displaystyle \mathrm{n} \geqslant 2$. Then $\displaystyle \sum_{n=1}^{15} a_n \cdot b_n$ is equal to \_\_\_\_.