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In Abaliba country there are twenty cities and two airline companies, Blue Planes and Red Planes. The flights are planned as follows:

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Given any two cities, one and only one of the two companies operates direct flights (in both directions and without stops) between the two cities. Furthermore:

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There are two cities A and B between which it is not possible to fly (with possible stops) using only Red Planes. Prove that, given any two cities, a passenger can travel from one to the other using only Blue Planes, making at most one stop in a third city.

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