Consider the function $f:\mathbb{N} \rightarrow \mathbb{N}$ , where $\mathbb{N}$ is the set of natural numbers. Suppose $f$ satisfies the following properties:

- $f(1)=1$
- $f(a+b+ab)=a+b+f(ab)$ , for $a,b \in \mathbb{N}$

Find the value of $f(2015)$ .

The answer is 2015.

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Note that $f(x)=x$ satisfies the constraints, so $f(2015)=\boxed{2015}$