Let A and B two countries which inlude exactly 2011 cities.There is exactly one flight from a city of A to a city of B and there is no domestic flights (flights are bi-directional).For every city X (doesn't matter from A or from B), there exist at most 19 different airline such that airline have a flight from X to the another city.For an integer k, (it doesn't matter how flights arranged ) we can say that there exists at least k cities such that it is possible to trip from one of these k cities to another with same airline.So find the maximum value of k. combinatorics unsolvedcombinatorics