Horses can start or end from/to any cell not having a tree on the field. Sam must be present on a cell from where he can see both the horses all the time. Sam can see only in horizontal or vertical direction. code example

Example: one space diagonally in java

class Point {
    private:
        int x, y;
    public:
        Point(int i, int j); // x = i; y = j;
        int GetX();
        int GetY();
        void SetX(int i);
        void SetY(int j);
        void SetPoint(int i, int j); x = i; y = j;
}