Let n and ℓ be integers such that n≥3 and 1<ℓ<n. A country has n cities. Between any two cities A and B, either there is no flight from A to B and also none from B to A, or there is a unique two-way trip between them. A two-way trip is a flight from A to B and a flight from B to A. There exist two cities such that the least possible number of flights required to travel from one of them to the other is ℓ. Find the maximum number of two-way trips among the n cities. combinatoricsflightsgraph theorycombinatorics unsolved