Formula to calculate average difference between dates.

Hello,

I've been trying to calculate average difference between two date columns without using any helper columns. Example workbook added.

The whole row should be ignored if column B or C contains a blank cell

Column A: Item name
Column B: Date when the item was opened
Column C: Date when the item was closed

As far as i have achieved is this :

{=AVERAGE(IF(ISNUMBER(B1:B100)*ISNUMBER(C1:C100),C1:C100-B1:B100))}

although it returns DIV/0 error.

Thank you for any help and tips.