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Answer is A+B factorial. Somehow I thought it is double factorial for a long time and thus got stuck on this problem (which also costed me F T_T).

In the first announcement, there is a white text giving the answer.

I actually don't know how to solve this problem. However, some guy submitted $$$94$$$ times before the submission limit got reduced to $$$20$$$. Thus, I guess that the answer should be around $$$94$$$. I tried roughly $$$15$$$ values around $$$94$$$ and found the answer.

Thanks BohdanPastuschak for making the $$$94$$$ submissions :)

Note that A000040 on OEIS are the primes while A072538 is a modified list of primes. Output that sequence.

On the first line, output the names of the N students, in any permutation. On the second line, output the N floors where the students live, in any permutation.

Just do that lol

Two-letter codes are country codes and you need to output the phone codes. Note that there are country codes that do not begin with the ones given in the map, so I parsed all of them from some random list online.

My thoughts during contest: Solve the problem (while making sure you take care of the corner cases). Note that the last sample is wrong (a hoax).

Actual solution: You don't need to minimize the number of days :face_palm:

Download the image, and floodfill (with say Paint) to read the hidden text.

Output $$$1 + 1 + 2 + ... + n$$$. See this part of statement.

"In other words, after he pays a to visit a-th city and goes back to the first city, he wants a path that costs him the least."

Actually I just started with one deterministic strategy that beats the first 2 cases, and another that beats the last, and select the first one with $$$\frac{2}{3}$$$ probability and the last one with $$$\frac{1}{3}$$$ probability. It should pass in expected $$$7$$$ tries.

After seeing actual solution: Wow, so it really is random after all lol.

Output $$$0$$$. I just saw the sample and YOLOed.