I'm honored to introduce you to the article series, Arpa's problem of the month, hosted by Topcoder.
Every month, there will be a problem with complete explanations and solutions for one of the problems of the month. Here is the problem of April. Feel free to ask me questions and suggesting edits.
Help me by proposing a problem for the next month. In the May contests, look for a challenging problem that you would like to have complete explanations of it.
P. S. "Arpa's problem of April" is something rhythmic, isn't it?