Given the Sturm-Liouville operator
$ - \frac{d^{2}}{dx^{2}}y(x)+y(x)q(x)=zy(x),$
my question is how to use spectral data to obtain $ q(x) $ inside the last equation by the Gelfand-Levitan-Marchenkio method
$ q(x)= 2 \frac{d}{dx}K(x,x)$
for the case of this problem with even potential $ q(x)= q(-x) $ or on the half line $ [0. \infty) $. I know how to get $ q^{-1}(x) $ but i would be more interested in getting $ q(x) $ instead.