Derf

Derf is a fake number between 5 and 6. It was invented by Carly Shay.

In the episode iTwins, Carly sees hidden camera footage of Chuck Chambers beating up Spencer Shay, her older brother. To get revenge on Chuck, she makes up the number and tricks him into thinking it's part of the new "International Math Laws" that were recently passed by the U.N. Chuck uses the number during his math test and fails it. Therefore, he was forced to go to Camp Add'emup for the whole summer by his dad, Earl Chambers. It's also Fred backwards.