Question 108
In Leonardland, each road has one-way movement, connects two cities and does not pass through another city. The Statistics Department calculated for each city $A$ the total number $f(A)$ of citizens in the cities, to which the roads from $A$ lead, and the total number $g(A)$ of citizens in the cities, from which the roads lead to $A$. Prove that there exists a city $A$ with $f(A) \ge g(A)$.