Circle

After the coordinate axes are rotated through an angle $\frac{\pi}{4}$ in the anti-clockwise direction without shifting the origin, if the equation $x^2+y^2-2 x-4 y-20=0$ transforms to $a x^2+2 h x y+b y^2+2 g x+2 f y+c=0$ in the new coordinate system, then

$$ \left|\begin{array}{lll} a & h & g \\ h & b & f \\ g & f & c \end{array}\right|= $$