Calculate with multiple tables

I have set up a report that required that i link 5 tables. Each table is responsible for an inventory count.

Main table is item, that all tables are related to

1. table  1 is "inventroy_Qty displays current inventroy amount
2. table 2  is "PO_Qty" displays the quantity on purchase order
3. table 3  is " Prod_Qty" displays the quantity on a production order
4. table 4  is "Sales_Qty" displays the quantity on sales order (not shipped)

the problem is that i need to calculate what is available to be sold

to accomplis this i am using the follwing formula in excel

(inventory_qty + po_qty) - (Prod_qty + Sales_Qty) = Available_Qty.

how can i perform this calculatin in Power BI. using BI desktop.