Happy holidays!
You might have stumbled across the largest online encyclopedia of integer sequences (OEIS) when finding the solution to a competitive programming problem. OEIS helps a lot of researchers and students around the world. The website of OEIS (http://oeis.org/) is supported by donations to a not-for-profit organization, The OEIS Foundation. They conduct appeals once every year where they ask people to donate to them. So, if you can afford, consider donating to them this year. The link to donate is available on their homepage.
PS: I am in no way associated with OEIS, and I won't receive any benefit from the funds you're going to donate. I just want to see OEIS growing. That's why I posted this.
Those who solved today's D using OEIS should consider donating :)
How using OEIS?
Bruteforce the answer f(n). Then write the sequence "n * n! — f(n)" for n = 1, 2, 3, 4, 5 in oeis. You will get the solution.
Wrote a brute, observed that every term is of the form n!*(n-2)*x, found pattern of x on OEIS.
I wrote bruteforce, I noticed nothing helpful, I got nothing. :P
early terms were coming out to be: 9,56,395,3084,26621.... 9=3!*1 + 3, 56=4!*2 + 8, 395=5!*3 + 35, 3084=6!*4 + 204 and so on.... and found the pattern for these adding terms(3,8,35,204....) on OEIS.