Problem: Telephone calls between San Francisco (SF) and Boston (BO) travel through the network shown in the diagram. Calls between these two cities can be routed through Denver (DN), Phoenix (PH), Houston (HO), Chicago (CH), or Atlanta (AT). The number of telephone lines between each location is indicated by the numbers on the arcs connecting each pair of cities. For example, there are 900 lines between San Francisco and Denver. What is the maximum number of calls that can take place between San Francisco and Boston at any one time?