How can I expand $\sum _{m=1}^{n-1} \prod _{k=m+1}^n x_k x_m$

How can I expand $\sum _{m=1}^{n-1} \prod _{k=m+1}^n x_k x_m$

$n \geq m\geq 1$
How can I expand this expression
$$\begin{align*}\sum _{m=1}^{n-1} \prod _{k=m+1}^n x_k x_m\end{align*}$$
for example $n=5$.