MathNet 2007-part-01 — 187 problems (EN–VI) Question 133

Given four fixed points $A(-3, 0)$, $B(1, -1)$, $C(0, 3)$, $D(-1, 3)$ and a variable point $P$ in a plane rectangular coordinates system, the minimum of $|PA| + |PB| + |PC| + |PD|$ is ______.