The Jacobian for this transformation is
with determinant , hence the area element becomes
Then the integral becomes
where is the unit circle,
so that
Now you could evaluate the integral as-is, but it's really much easier to do if we convert to polar coordinates.
Then
By definition, we have
So, we have to solve two different equations, depending of the possible range for the variable. We have to remember about these ranges when we decide to accept or discard the solutions:
Suppose that
In this case, the absolute value doesn't do anything: the equation is
We are supposing , so we can accept this solution.
Now, suppose that . Now the sign of the expression is flipped by the absolute value, and the equation becomes
Again, the solution is coherent with the assumption, so we can accept this value as well.