FormAPI - random value problem

Hello,

I need help with this interesting formAPI problem:
I need something like this:
1. generate a random number, and display the value in the form.
2. let the user submit the form.
3. retrieve that value in the submit function.

So what I tried is to store the value in a form element:

form['random_number'] = array (
  '#type' => 'value',
  '#value' => generateRandom(),
); 

Then, in the submit function I try to do $form_state['random_number'].
But this does not seems to work. And instead of getting the original value that was displayed in the form, I get a new generated random.
This is I think because the form is being built before the validation, and overrides the value with a new random.

How can I solve this, retreiving the original random number that was used in the first building of the form?
Thanks.