For example, the nurse scheduling problem is concerned with scheduling a number of employees with typical constraints such as rotation of shifts, limits on overtime, etc. The travelling salesman problem is concerned with scheduling a series of journeys to minimize time or distance. Some of these problems may be solved efficiently with linear programming, but many scheduling problems require integer variables. Although efficient algorithms exist to give integer solutions in some situations (see network flow models), most problems that require integer solutions cannot yet be solved efficiently.