Simple game of darts

There’s only two rules to this game:

1) You must get to 501 points
2) Your last shot must be a double (not as easy as it sounds)

The points are 1-20, and if you get in the outer ring it’s worth 2x the points. The inner ring is worth 3x the points. The red of the bulls-eye is worth 25, and the black center of the bulls-eye is worth 50.

http://appshopper.com/games/darts-501-on-myhip