Let a_n=∫_-1^n(1+x/2+x^2/3+… ….+(x^n-1)/n) d x for every n ∈ N. Then the sum of all the elements of the set {n ∈ N: a_n ∈(2,30)} is \ \ \ \.

Let $\displaystyle a_n=\int_{-1}^n\left(1+\frac{x}{2}+\frac{x^2}{3}+\ldots \ldots .+\frac{x^{n-1}}{n}\right) d x$ for every $\displaystyle n \in \mathbb{N}$. Then the sum of all the elements of the set $\displaystyle \left\{n \in \mathbb{N}: a_n \in(2,30)\right\}$ is \_\_\_\_.