Chefs Game

Chef is playing a game where he has $N$ points on the straight line - positions of the sugar cubes and $M$ points on the same line - positions of ants. All ants work the same way, an ant can sense sugar cube within range of $R$ .Your task is to find $minimal$ $R$ such that every cube is sensed by either of the ant, i.e. for each sugar cube there is at least one ant at the distance which is not mor