The Tower of Hanoi
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[The Tower of Hanoi](http://en.wikipedia.org/wiki/Tower_of_Hanoi) is a mathematical game or puzzle. It consists of three rods, and a number of disks of different sizes which can slide onto any rod. The puzzle starts with the disks in a neat stack in ascending order of size on one rod, the smallest at the top, thus making a conical shape.

The objective of the puzzle is to move the entire stack to another rod, obeying the following simple rules:

* Only one disk can be moved at a time.

* Each move consists of taking the upper disk from one of the stacks and placing it on top of another stack i.e. a disk can only be moved if it is the uppermost disk on a stack.
* No disk may be placed on top of a smaller disk.

* With three disks, the puzzle can be solved in seven moves. The minimum number of moves required to solve a Tower of Hanoi puzzle is 2<sup>n</sup> - 1, where n is the number of disks.

###Challenge

Write a function that takes the number of discs, performs the moves needed to rearrange the tower on another rod, and returns the number of steps.