Minimal value of $\sum_{1\leq i<j\leq 2014}a_ia_j$ where $a_i=\pm1$

Hint: $\displaystyle \left(\sum_{i=1}^{2014}a_i\right)^2=\sum_{i=1}^{2014}a_i\sum_{j=1}^{2014}a_j = \sum_{i=1}^{2014}a_i^2+2\sum_{1\leq i<j\leq 2014}a_i a_j.$