This is probably a really easy question, but the material I've been given doesn't explain things like this at all. I know the definitions of the expected value, ACF etc, but I'm not sure how to apply it to this situation.

$\displaystyle X[n]=\pm 1$ is a discrete stochastic process. Determine the ACF, i.e. $\displaystyle E(X[n_1]X[n_2])$.

The process is SSS (strict sense stationary) if that helps.