In an NxN forest, position as many hunters as possible, such that there are no more than two hunters in any straight line, and no hunter can see another because there is at least one tree between any two hunters. After maximizing the number of hunters, try to minimize the number of trees. Each tree and each hunter take up one unit square in the forest. For example, in a 3x3 forest, four hunters can be placed. In a 5x5 forest, at least five hunters can be placed. Find the maximum number of hunters for N=4,5,6,7,8,9.