I would like to setup Accounting as follow to get Gross Profit

Profit and Loss

1. Revenue

      - Product Sales
   

2. Cost Of Sales

      - Opening Stock
      - Purchases
              * Raw Mat 1
              * Raw Mat 2
      - Closing Stock
   

=> Opening stock is a value of stock from previous month/period

=> Closing stock is a value of stock from current month (after closing)

Gross Profit = Revenue - ( Opening Stock + Purchases - Closing Stock)

I have no issue with Revenue, Purchases BUT still cannot set the opening stock and closing stock ( because I think much related with inventory).

Anyone have some ideas since I'm not well versed in accounting.

Amin