Average of text boxes with values

Hi,

I'm trying to write a custom calculation script for a text box that calculates the average value of the numbers entered into several other text boxes, but also omits from the denominator any text boxes that don't have a value.  For simplicity sake, I named the text boxes that will contain the numbers going into the average calculation A1, A2, etc. through A11.  So for example if 9 of the text boxes have values, it would be the sum of values divided by 9 instead of 11.  I looked at a couple of code examples on this board and couldn't get them to work correctly.  Could anyone show me how to calculate this average?

Thank you.