Killing Gs

There are the big enemies for all chefs, that is, Gs (the plural form of G). Chef Ciel find N Gs in her restaurant. Ciel is very scared, and she is going to kill the Gs by using insecticides. Here M insecticides are available, and the price of the j-th insecticide is Cj, and she can use the j-th insecticide for all N Gs if she buys it. Unfortunately Gs may have resistance to insecticides, so Gs ma