SMO 2008 Open Round 1 Question 10

For a positive integer $n$, let $N_n=\{1,2,3,\ldots,n\}$. Assume $n\ge10$. Determine the maximum $n$ such that $\displaystyle\min_{\substack{a,b\in A\\a\ne b}}|a-b|\le10$ for every $A\subseteq N_n$ with $|A|\ge10$.