There are some nails on a board, and we try to put a rope to include all the nails. We assume that the area of a single nail is zero, so the position of the nails can be present as a coordinate on the board. Furthermore, we want to minimize the num- ber of nails to fix the shape of the rope. For Example, if the coordinates of nails are (0,0),(2,1),(0,2),(2,4),(0,7),(2,8), we only need 4 nails to fix the rope. They are (0,0),(2,1),(2,8),(0,7), although the rope has touch all the 6 nails.