MathNet 2014 Question 71

Each of the numbers $x_1, x_2, \dots, x_{2014}$ is $1$, $0$, or $-1$. What is the minimal possible value of the sum of products of all the pairs of those numbers, i.e. the sum of all $x_i x_j$ for $1 \le i \lt j \le 2014$? (USSR 1965)