creating a vba code for my inventory

Hello

In my workbook, I have 2 sheets. One is Sheet 1 and the other is Sheet 2.
In sheet 1 I have the function below in Sheet 1 cell A2 that counts the sku number "JJ-KI-OPED" from Sheet 1 cell B2 if Sheet 2 has it typed in any D:D cell.
=IF(LEN(B2),COUNTIF(Sheet2!D:D,"*"&B2),"")

Although the function works like a charm, it's inefficient in telling me how many items I have left instock.
I would very much like to have a vba code that does the same thing as my function but instead of having Sheet 1 cell A2 return the value of how many sku numbers from Cell B2 where found in cell A2. I want the returning value to be subtracted by any number I typed in cell A2 and the result in C2.