Question 3
Let $a_1,a_2,\ldots,a_n$ be positive integers, not necessarily distinct but with at least five distinct values. Suppose that for any $1\le i\lt j\le n$, there exist $k,l$, both different from $i$ and $j$, such that $a_i+a_j=a_k+a_l$. What is the smallest possible value of $n$?