Let the circle $C_1: x^2+y^2-2(x+y)+1=0$ and $C_2$ be a circle with centre $(-1,0)$ and radius $2$. If the line of the common chord of $C_1$ and $C_2$ meets the $y$-axis at the point $P$, then the square of the distance of $P$ from the centre of $C_1$ is: