How to keep an averaging cell empty until all the input cells are filled

I am making a spreadsheet for recording student grades.

I want the cell that is averaging the grade for semester 1 to remain empty until the grades for both quarter 1 and 2 are entered. Column B is Quarter 1 grades and column D is quarter 2 grades. Column F averages a student's B and D quarter grades. I want the average column F cell to remain empty until BOTH B and D cells are entered.

Should be simple but couldn't find a formula to make it work...

Am currently using the formula =average(B2,D2) for the semester average and it shows the value of B as soon as it is entered while D is still empty

Found it on another forum. Sweet! Knew it couldn't be that difficult

=IF(OR(B2="",D2=""),"",AVERAGE(B2,D2))