Let the circle \( C_{1}: x^{2}+y^{2}-2(x+y)+1=0 \) and \( C_{2} \) be a circle having centre at \( (-1,0) \) and radius \( 2 \). If the line of the common chord of \( C_{1} \) and \( C_{2} \) intersects the y-axis at the point \( P \), then the square of the distance of \( P \) from the centre of \( C_{1} \) is:
