cf_1621d The Winter Hike

All snow in the bottom right quarter of the grid have to be removed, so let’s focus on the top right and bottom left quarters. A key observation is that, if any of the eight corner cells of the top right or bottom left quarters (painted green in the image below) is NOT covered with snow, then all people can be moved to the bottom right quarter and no other cells need to be cleared. (How to do that?)

On the other hand, if all the green cells are covered with snow, it is impossible to move all people to the bottom right quarter. Consider the four corner cells in the upper left quarter (painted yellow in the image below). When the earliest move of two of the people initially in the yellow cells happens, one of the two moved people must go to a green cell, and that green cell must be clear of snow.