Complicated algebra terms

This is a little complicated, i'll try to explain as best i can.

Say I have n different numbers, $\displaystyle z\sb{1}, z\sb{2}, ... , z\sb{n}$

I wish to find the summation of every possible way of multiplying $\displaystyle x$ different numbers together.

For clarification: if $\displaystyle n=3, x=2$ I would want:

$\displaystyle z\sb{1}z\sb{2} + z\sb{1}z\sb{3} + z\sb{2}z\sb{3}$

If $\displaystyle n=4, x=3$ i would want:

$\displaystyle z\sb{1}z\sb{2}z\sb{3} + z\sb{1}z\sb{2}z\sb{4} + z\sb{1}z\sb{3}z\sb{4} + z\sb{2}z\sb{3}z\sb{4}$

Basically, I just need a function $\displaystyle F(x,z\sb{1},z\sb{2}, ... ,z\sb{n})$ that gives this result.