Conditional multiplication?

Hi All,

I have attached an example sheet with three columns. I'm trying to get a result in column B that is the product of the corresponding cell in column C and the cell in column C where the value in the row in column A reduces by 1. Terribly explained I know but in the example I would like cell B22 to be 4 as that is C22 multiplied by C18 (A18 being the cell where the number reduces from 3 to 2).

Let me know if this isn't clear and you need further descriptions! Thanks