Statement

Fearsome William is still trying to hide from the police, who have stepped up the game. In a very long (L meters) street, the police officers have set up N checkpoints, to first see and then stop the criminal. With the help of binoculars, the officers at each checkpoint are able to see up to M meters away in both directions. Formally, this means that a checkpoint located at D[i] meters from the beginning of the street can see people from D[i]-M meters (included) to D[i]+M meters (included), measured from the beginning of the street. William is no longer by car and has no other choice to walk a little bit and spend the night hiding somewhere along the street with all the checkpoints, at any point from 0 to L (both included). He wants to minimize the number of checkpoints from which he can be seen: in one of the possibly many optimal positions, how many checkpoints will he be visible from?