Suppose n≥3 is an integer. There are n grids on a circle. We put a stone in each grid. Find all positive integer n, such that we can perform the following operation n−2 times, and then there exists a grid with n−1 stones in it: ∙ Pick a grid A with at least one stone in it. And pick a positive integer k≤n−1. Take all stones in the k-th grid after A in anticlockwise direction. And put then in the k-th grid after A in clockwise direction. combinatoricsAZE CMO TSTAZE EGMO TST