Problem 564

10. Mom decides to take Xiao Hua on a car trip to 10 cities during the holiday. After checking the map, Xiao Hua is surprised to find: among these 10 cities, any three cities are either all connected by expressways, or only two of the cities are not connected by an expressway. Therefore, there are at least $\qquad$ expressways between these 10 cities. (Note: There is at most one expressway between any two cities)