In english tutorial of https://mirror.codeforces.com/contest/460/problem/A, it is given that answer can be calculated using formula
/predownloaded/7b/26/7b263e7a36986448eaf36de9cabc77477c0809ef.png , 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://mirror.codeforces.com/contest/460/submission/76206660