I have a row of fields in a repeating table.  Here is the layout:

Field 1  Field 2  Field 3 Field 4 Field 5  Field 6
 %          %        #        #        #       Total

I want to sum fields 3, 4 & 5 and then multiply them by field 1 and then
multiply all of that by field 2.  The formula I'm using works, but only if
you fill out both fields 1 and 2.  Sometimes, we don't want to put anything
in fields 1 or 2, but we still want fields 3-5 to be summed in the total
field.

This is the formula I'm using in the "total" field.  How can I adjust it so
that it works even when there are no values in fields 1 or 2?  Or just a
value in field 1, but not field 2, etc?

((field3 + field 4 + field 6) * field1) * field2