201
xkcd #3019: Advent Calendar Advent Calendar
(xkcd.com)
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(n*(n+1)/2 = 25*13 =325
Never mind the fact that the "correct" number of days on an advent calender is 24.
For one, it looks to me like the triangle numbers, which for triangle number T~n~ = (n+1) choose 2.
So for a correct nested advent calendar of 24 days, it's T~24~ = (24+1) choose 2 = 300
Also, it appears that they created the terminal function that does that in a very compact notation, so another correct way to write down this sum is "24?" Where n? = n + (n-1)?