Martians are invading the earth! Yes, it's not an illusion, it's the truth. Their attack are so quickly and unpredictable that few people realize it until many most buildings are destroyed. As a human being on the earth, you feel it's your duty to protect your homeland.
After scouting, you find there are some powerful weapons which are still working well on the land. They are the last hope to defeat the Martains, so all of them should be protected. And there are a few of defensive devices can be used. If you activate these devices in the order D1, D2, .., Dn, then there will be a laser line between D1 and D2, D2 and D3, ..., Dn and D1, which forms a polygon. Everything enclosed by the polygon or laid on the boundary can be protected well. Due to some physical reasons, nonadjacent laser lines can not intersect (even intersect at end points), or they will disappear and leave a hole in the polygon, which Martains can go through to destroy everything. Of course, no devices can be activated two or more times.
The final wave of Martains is near, so you need design a plan to construct our defence quickly. Activating a defensive device is very hard and time-consuming, so you should active as few as possible to protect all the weapons. You have the ability, and you have the duty, too.