AT-OEMO-FED1 2007 Problem 2

For every positive integer $n$ determine the highest value $C(n)$ , such that for every $n$ -tuple $(a_1,a_2,\ldots,a_n)$ of pairwise distinct integers

$(n + 1)\sum_{j = 1}^n a_j^2 - \left(\sum_{j = 1}^n a_j\right)^2\geq C(n)$