expected value from some points in continuous homogeneous spatial Poisson point process

Question

Let $n$ point are distributed as per a homogeneous spatial Poisson process of rate $λ$ in a square of side $2a$, and assume that $4$ fixed points are located at $(a/2,a/2)$, $(-a/2,a/2)$, $(a/2,-a/2)$ and $(-a/2,-a/2)$, when the center of the square is $(0,0)$. If the sum of the distance between each random point and 4 fixed points be a random variable called X. How can I find the expected value $E(X)$?