Finding growth when negative values occur.

Hi,

I wondered if anybody could assist with the following:

I am running the following formula to calculate growth over a series of values over a number of years:

=(J2/B2)^(1/8)-1

The problem is if J2 or B2 are -ve values the formula returns #NUM!
B2 is the first value.
J2 is the last value.

Is there a way to ask the formula to skip B2 and move to C2 (assuming this is +ve) and use this value instead.
Similarly wit J2, could the previous value be used by the formula?

It is easier to understand in the attached file where I have put the calculations in.
Maybe there is a different way of calculating growth over a period. If both values are -ve, then the resultant growth value is +ve which is incorrect.

Terry