What could it be?

The series can be written as:

$\large \Sigma_{n=1}^{2015} \frac{1}{\sqrt{n}+\sqrt{n+1}}$ ;

or $\large \Sigma_{n=1}^{2015} \frac{1}{\sqrt{n}+\sqrt{n+1}} \cdot \frac{\sqrt{n+1}-\sqrt{n}}{\sqrt{n+1}-\sqrt{n}};$

or $\large \Sigma_{n=1}^{2015} \frac{\sqrt{n+1}-\sqrt{n}}{(n+1)-n};$

or $\large \Sigma_{n=1}^{2015} \sqrt{n+1}-\sqrt{n}$

which telescopes to $\sqrt{2016} - 1 \Rightarrow 2016+1=\boxed{2017}.$