The problem is this, Given n (n <= 100) gardeners, m (m <= 500) flowers, and given the list of flowers that every gardener can take care of. What is the lowest number of gardeners who are needed so that all the flowers are kept by at least one gardener? A gardener can take care of various flowers.

I tried to think with flow, but I can not think how to build the network, someone can give me some hints?

sorry for my poor English