SRM 423 div1 500p

Today I have practiced in the problem of SRM 423 div 1 500pts.

After writen the same problem for three times, I finally got a 400+pts.

I am wondering how can those people can solve this kind of problems in less than half an hour while they have to read the description of the problesm and they also have to think, then to code.

It almost need to write the program right for the first time without a tiny mistake.

That's amazing.