Get Data Dependence Sum (Fungible :)

Problem Overview

• Compute the sum of all day indices x (0 ≤ x < n) such that there exists a positive integer k with floor(n/k) = x.
• Input: one long integer n. Output: a long integer sum. Constraint: 1 ≤ n ≤ 10^10.
• Real-world context: models data dependence in Amazon time-series analytics for items across days.
• From Amazon interviews; a coding interview problem and practice interview question.