Compute the genus of a curve with a flex point

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The genus of a smooth plane curve is $g=\frac{(d-1)(d-2)}{2}$ and I know that if the curve has $n$ nodes the genus decreases by $n$. What happens if the curve has singular (non ordinary) points? In particular I would like to know how the genus decreases in the case of an intersection point between the Hessian and the curve. I guess I have to count it at least one but maybe more, but I can't find any reference on this. I found something about the delta-invariant but I'm not quite sure about that.