I've been working on a puzzle for ages and i'm really quite proud of the solution:

You have 52 playing cards (26 red, 26 black). You draw cards one by one. A red card pays you a dollar. A black one fines you a dollar. You can stop any time you want. Cards are not returned to the deck after being drawn. What is the optimal stopping rule in terms of maximizing expected payoff? Also, what is the expected payoff following this optimal rule?

it took me two years to find the solution

it's here http://puzzles.nigelcoldwell.co.uk/ #14