ISL 2007 A2

Consider those functions $f: \mathbb{N} \rightarrow \mathbb{N}$ which satisfy the condition $$f(m+n) \geq f(m)+f(f(n))-1$$ for all $m, n \in \mathbb{N}$. Find all possible values of $f(2007)$. $(\mathbb{N}$ denotes the set of all positive integers.) (Bulgaria)