From what I can see of your solution, it looks like you're close. Most solutions fall within the 25-35 character range (the lowest being 20); I agree it's tougher than a 7kyu (I ranked it 6 myself), but it's not too hard; I learned the basics of how to do this just from observing other solutions to other problems on the site. There's no special math trick you're not already using that is necessary to pass; try cutting down the extra function you've created so you don't have to both declare and call it.. just embed the step in your solution :)

Edit: Yep, just cut your solution down to 38 chars by moving some stuff around. There is another "trick" (again, just a little insight problem, no math knowledge required) that would help you cut it down dramatically, but you don't need it to pass.