How many lines will there be in the truth table for a statement containing n letters?

I know:

2 letters is 4 lines

3 letters is 8 lines

From this i came up with

For "n" being EVEN --> $\displaystyle n^2$ is the number of lines

For "n" being ODD --> $\displaystyle n^2 - 1$ is the number of lines

I then tested it out.

It holds true for 4 letters (16 lines) but it does not hold true for 5 letters (26 lines).

Can anyone help me out with this?

Thank you