Let f(x) be a function such that f(x+y)=f(x) · f(y) for all x, y ∈ N. If f(1)=3 and Σ_k=1^n f(k)=3279, then the value of n is

Let $\displaystyle f(x)$ be a function such that $\displaystyle f(x+y)=f(x) \cdot f(y)$ for all $\displaystyle x, y \in \mathbb{N}$. If $\displaystyle f(1)=3$ and $\displaystyle \sum_{k=1}^n f(k)=3279$, then the value of n is