MathDB

In a country there are two-way non-stopflights between some pairs of cities. Any city can be reached from any other by a sequence of at most

100

flights. Moreover, any city can be reached from any other by a sequence of an even number of flights. What is the smallest

d

for which one can always claim that any city can be reached from any other by a sequence of an even number of flights not exceeding

d

2