Hello Codeforces.

We are introducing a new type of problem which may appear in future Codeforces rounds, called Communication problems!

In these problems, your program would be ran twice, with different input and output formats between the runs. All variables stored in the memory will be lost between runs. However, the information given to you on the first run may be important to you for completing the second run correctly. Therefore, one of the key challenges in these types of problems would be to find a way to use the limited amount of output you are given in the first run to communicate information to the second run. Below shows a flowchart.

Flowchart for communication problems

In these problems, time and memory limits for both runs would be kept separately. For example, if a problem has a time limit of $$$2$$$ seconds, you will only get the Time Limit Exceeded if program runs for more than $$$2$$$ seconds on one of the runs, but not if both runs takes $$$1.5$$$ seconds.

It is also possible that a problem is both interactive and run-twice. There may be interaction on either run of your program. For these types of problems, it is especially important to read the problem statement carefully to ensure you are getting the input and interaction format correctly, especially regarding whether you must flush your outputs.

To give participants a feel of these new types of problems, we will hold Testing Round 20 (Unrated, Communication Problems), which will start on Nov/03/2025 17:35 (Moscow time). You will be given $$$1$$$ hour to solve $$$3$$$ run-twice problems. The problems are authored by cry, yse, SpyrosAliv, and chromate00. One problem on the set will be interactive, so you are recommended to read the guide for interactive problems if you are unfamiliar with these types of problems. Of course, this round will be unrated. Good luck, and I hope you will have fun with the new type of problem!

Edit: The contest will be in ICPC format with no pretests and hacks disabled.