Slope calculation : Find last valid number automatically

Hallo everyone,

I am trying to do following operation. Kindly look at the attached excel sheet for simple example.

It is regarding "Slope" calculation of line automatically depending on length of the data (that also considers the last valid number)
I need a single formula, which obeys all the 5 cases shown in "colored" column in the attached file (i.e) Slope .

slope_automatic.xls

I wanted to find a formula which can automatically perform "Slope" of the line calculation.

It should consider cases like : Skipping of the first or/and last number IF they are "Non-Available"number. Also depending on length of dataset (x and y set), the slope calculation should be automatically updated, depending on last valid cell address/value.

Please help me. I am doing it manually and I dont know how to use a valid formula to automate it.

regards,

- Excel'less excel user