Game Theory (possibly redundant)

[LetYouDown]

Tried to search...but no avail. I'm sure someone has this completely solved somewhere as it's not overly difficult. Someone posed this question to me:

"My buddy brought up this math puzzle where you have 20 items on the table, i.e. sugar packets. Now you can pick up 1 or 2 on your turn and each person picks until someone gets the last 1 or 2 on the table. I was wondering if you could figure out mathimatical relationship needed to calculate your next move when picking?"

My initial suspicion is that going first is a disadvantage and that you never wanted your opponent to have a turn when there are 4 or 5 items left. So you'd never want a turn when there are 6 items left. So you'd never want him to have a turn when there are 7 or 8 items left, etc. etc.