25 minutes.

A hard number doesn't really matter, but you should feel frustrated/exhausted before reading the solution, when you have exhausted all possiblities then look at the solution. When you read the solution take note of why you couldn't solve the problem, usually it is one these few things:

• new technique that you did not know about
• new pattern/knowledge that arises from the setup (bitwise properties or permutation properties or maths)
• you failed to make an observation.

first two are easy to fix (just learn them). In the third case try to quantify why you didn't make that observation and what you could have done or in which direction you could have thought in to make that observation.

This is what I used, hopefully it works for you too.

10^9+7 minutes. That 10 hour solve is so rewarding icl, and its fun too.

This is what I used, hopefully it works for you too.