today i have created this hard problem (at least for me). i am sure you find it interesting.

please read and solve :)

Gym link = https://codeforces.com/contestInvitation/bcfb569f9f1526e7330bc671f1b10a0d7aaa587e

I am solving a problem in which at some point i need to find answer for this sequence, when $$$n$$$ is known.

$$$n\times(n + 1) \; + \; (n - 1)\times n \; + \; (n - 2)\times (n - 1) \; + \; (n - 3)\times (n-2) \ldots 1\times 2$$$

it has $$$n$$$ terms.

Ans = $$$\frac{n \times (n + 1) \times (n + 2)}{3}$$$

thanks sam_2912 for help.

Yesterday I created problems using the Polygon system.

there is one Problem with two Parts($$$n \le 400$$$ and $$$n \le 2\times10^5$$$)

i would love if you see and solve the problems!

gym link : https://codeforces.com/contestInvitation/ef9be4fa454826fa1bcc152a56c098d0b5c260f8

UPD : I have also Uploaded Editorial, Link : Editorial