Formulate the following problem as an integer program: A set of n items are to be repaired in a factory. Item i takes time t_i to repair and requires W_i workers working continuously. It arrives at time a_i and it must be finished by time d_i. The problem is to find a repair schedule that minimises the total number of workers needed. (When a worker has finished working on one job, he/she can work on another job).