On the time complexity of an algorithm

The difficulty of estimation the time complexity of an algorithm in the worst case is not always the same as the mathematical complexity of the solved task. In this paper we demonstrate a mathematically clear task proposed in a programming contest for Grade 6 students. The task has efficient algorithmic solution but estimation of its time complexity in the worst case is really difficult. We provide corresponding reasoning of the estimated complexity and propose some explanation of it understandable for pupils of that age.