Although I found answers on calculating angles from vectors, I didn't find a specific way to calculate angles between line-segments that do not necessarily touch each other (I say "not necessarily because I will apply to different cases).
See the figure below:
Consider that I know the 2D position of all 4 red points. I am using C# here. What would be the cheapest way of calculating the angles formed by line-segments that do not touch each other, such like the ones above?