Good Joke!

Vadim and Roman like discussing challenging problems with each other. One day Vadim told his friend following problem: Given N points on a plane. Each point p is defined by it's two integer coordinates — px and py. The distance between points a and b is min(|ax - bx|, |ay - by|). You should choose a starting point and make a route visiting every point exactly once, i.e. if we write down numbers o