There is so much wrong here, I don't know where to start! You need to read up about using formulas in Excel.
For one thing, there are no "variables", but rather cell addresses. For example, to use the value of R, which you put in cell C2, you need to reference it with $C$2 (the dollar signs mean that neither C nor 2 will change when you copy the cell, it is an absolute address). Another example, in G4, to use the value of n, you reference $E4. There is no dollar sign in front of the 4 because you want the formula in G5 to read $E5 instead.
Also, why do you take n0 as 5? Why do you have a n-1 in the formula?