Nutrition Cost

There are $N$ different food items, each providing an essential vitamin $A_i$ and costing $B_i$. There is an integer parameter $C$ which represents the value of a single vitamin to you. You will buy some of the food items to maximize $C \cdot X - Y$ ($C$ multiplied by $X$ minus $Y$), where $X$ represents the number of **distinct** vitamins eaten, and $Y$ represents the total cost of all the food