Constructing logical sentences that involve negative integers over the nonnegative integers
Any integer $x \in \mathbb{Z}$ can be expressed in the form $x = x_+ - x_-$ where $x_+ \in \mathbb{N}$  and $x_- \in \mathbb{N}$ (i.e., every integer can be expressed as the difference of two natural numbers; and conversely, the difference of any two natural numbers is always an integer).

Try using that to formulate your statement.  You'll need to work out how to define the $\le$ operator, but that's a separate issue.