I am trying to find the average case complexity of a sequential search. I know that the value is calculated as follows: 

Probability of the last element is $\frac{1}{2}$
Probability of the next to last is $\frac{1}{4}$
Probability of any other elements is $\frac{1}{4n-8}$

I can assume the I have a list of n $\geq$ 3. How Would I go about solving this?