A firm is considering four sites for locating its warehouses. The cost of locating a warehouse $($Wi$)$ at site i is $Ki$.$ There are nine retail outlets $($Rj$),$ each of which must be serviced by at least one warehouse. As shown in the network, it is not possible for any one warehouse site to supply all retail outlets.

If the problem is to determine the location of the warehouses such that the total cost is minimized, formulate the constraints for this problem. Likewise, determine the minimum number of warehouses to open.