Minimal network between four points

Choose four cities in the map by moving points \(A\), \(B\), \(C\) and \(D\) to the desired location.

(In order to work, the points \(A\), \(B\), \(C\) and \(D\) should be chosen clockwise.)

To see the minimal (shortest) network of roads that links those four cities, please move the red dot until the points \(X\) and \(Y\) overlap. Only then the minimal network will appear automatically.

(The following applet was produced with the help of JavaSketchpad)