Suppose that you start with n = 0.

You are given a huge pile of checklists, all of which are photocopies of the same thing.
The contents of these checklists are:
>step 1: add 1 to n
>step 2: subtract 1 from n
>step 3: add 1 to n
>step 4: subtract 1 from n

You job is to perform all steps in all checklists in this manner:
>draw a random checklist
>perform the next step on the checklist, then cross it out
>put the checklist back into the pile
>draw a random checklist, repeat

Prove that n will never dip below zero.

--

I am fucking lost. Where do I even start with this problem?