Let p be prime and let $Y_p = 2^p − 1. $ Show $Y_p$ is either prime or 2-pseudoprime.

Question

Let p be prime and let $Y_p = 2^p − 1.$ Show $Y_p$ is either prime or 2-pseudoprime.

I have shown $Y_p-1$ is a multiple of $p$ and don't know where to go with this.