ISL 2015 A3

Let $n$ be a fixed positive integer. Find the maximum possible value of $$\sum_{1 \leqslant r<s \leqslant 2 n}(s-r-n) x_{r} x_{s},$$ where $-1 \leqslant x_{i} \leqslant 1$ for all $i=1,2, \ldots, 2 n$. (Austria)