Write a Python function distance(x1, y1, x2, y2)
that
returns the distance between the specified two-dimensional
points, (x1, y1) and
(x2, y2). The distance, \(d\), between two
points can
be calculated with the equation:
$$d = \sqrt{(x_1 - x_2)^2 + (y_1 - y_2)^2}$$