How can I apply combinatorics to calculate the number of intersections between two columns of points? In the photo below let x be the number of point in each column, for x=2 we have 1 intersection, for x=3 we have 7 intersections and for x=4 we have 27 intersections. I'm asking because I'm not well versed in combinatorics, so I thank you in advance if you'll help.
This is entry A159065 in the OEIS.