Question 86
In a country there are $N$ cities and among some of them there are (two-way) flight connections. Each flight connects exactly two cities. No city is directly connected to every other city. For any two cities $A$ and $B$ there is exactly one way to fly from $A$ to $B$ using at most two flights.
Prove that $N - 1$ is a square of a positive integer.
Prove that $N - 1$ is a square of a positive integer.