I tried Solving ICPC India Online prelims Question D but I am not to find the correct logic

https://www.codechef.com/ICPCOL2025/problems/ICPC25P4TMP

I Would like to hear what was the logic from the people who solved this

How can we create our own problems on Codeforces for a private group or in the Gym? Do we need to meet any eligibility criteria to create such problems?