or

In a certain city there are n straight streets, such that every two streets intersect, and no three streets pass through the same intersection. The City Council wants to organize the city by designing the main and the side street on every intersection. Prove that this can be done in such way that if one goes along one of the streets, from ts beginning to its end, the intersections where this street is the main street, and the ones where it is not, will appear in alternating order.