I recently came across this blog post and decided to try some of its problems. I am stuck on this problem. I can think of a naive O(n^2) solution by iterating through all pairs but do not see any redundancy in this solution. I cannot see a way to make it sub-quadratic (other than using the fact that 1 ≤ ai ≤ 2.5 * 10^6, maybe?). Any hints would be appreciated.