[lostcolony]

I always understood it just by saying "Okay, if we take the last item and the first (so n+1), and then the second to last item and second item (so n-1 + 2 = n + 1), etc, we get to the middle in n/2 times. If it was odd, we have an extra n/2. But trying it out on a few series, and it's obvious it works out each way, odd or even, to n/2 * (n + 1).