worked at least two months....

I think it is a hard problem,harder than some rating>1e4 problems...

How did you get the skill to solve such a hard problems?
I know generic patterns to effective skill training
Interested in particular case

Edited by author 08.11.2017 21:17

Just stop solving easy problems,and trying harder one.. I start solving hard problems since
year 2010 practicing at    http://poj.org  solving AC<100 problems

and at year 2013 I start solving topcoder Div I 1000pts.. later I found codeforces and start solve fewest AC problems..

There are also some Chinese oj hardest problems is very hard such like

http://www.lydsy.com/JudgeOnline/

http://uoj.ac/

Yeah, solving hard is very helpful, I felt it
At some point I noticed that I am just not able to solve some harder problem
Because it takes very much time to solve
So even if trying whole day then cannot solve
Maybe whole week is enough
But very rarely have an opportunity to solve a problem the whole week
Typical have only three hours per day
But it's at cost of sacrificing important things
So I feel like I cannot improve further

Yeah, solving hard is very helpful, I felt it
At some point I noticed that I am just not able to solve some harder problem
Because it takes very much time to solve
So even if trying whole day then cannot solve
Maybe whole week is enough
But very rarely have an opportunity to solve a problem the whole week
Typical have only three hours per day
But it's at cost of sacrificing important things
So I feel like I cannot improve further

On spoj.com I have lot of tasks in my TODO list...

you can choose some easier than hardest problems but harder than medium problems if you don't have enough time to practice

Why do you consider it a hard problem? It felt quite straightforward to me

Timus has very few constructive problems (at least with difficulty >= 2000), so this problem may seem hard compared to others if constructive problems are not your strong point