In english tutorial of https://codeforces.com/contest/460/problem/A, it is given that answer can be calculated using formula

, but I am not able to understand how we reached at this formula. Please help me in this.

Here is my accepted submission , but my solution will exceed time limit if value of n is very large. Fortunately in this question input size limit is very small, hence my solution works.

https://codeforces.com/contest/460/submission/76206660