The figure shows a blue line parallel to the $x$-axis through $(0, 1, 1)$ and a red line parallel to the $y$-axis through $(-1, -3, 3)$, with lengths in metres. A cable car designer needs the shortest distance between the two lines. What is it?

Fig. 1: A three-dimensional picture with the x, y and z axes. A blue line is parallel to the x-axis and passes through the point (0, 1, 1). A red line is parallel to the y-axis and passes through the point (-1, -3, 3). A green vertical segment joins the two lines and is labelled with the height difference.