Some of the n+1 cities in a country (including the capital city) are connected by one-way or two-way airlines. No two cities are connected by both a one-way airline and a two-way airline, but there may be more than one two-way airline between two cities. If dA denotes the number of airlines from a city A, then dA≤n for any city A other than the capital city and dA+dB≤n for any two cities A and B other than the capital city which are not connected by a two-way airline. Every airline has a return, possibly consisting of several connected flights. Find the largest possible number of two-way airlines and all configurations of airlines for which this largest number is attained. geometrygeometric transformationcombinatorics proposedcombinatorics