Suppose we choose the point ~P~ in which the angle is ~90~ degrees. Because the legs of the triangle must be parallel to the coordinate axes, then one of the other two points must have the same x-coordinate as ~P~, and the other must have the same y-coordinate. The exact number of triangles is ~[freq_x(x_P)-1] \cdot [freq_y(y_P)-1]~.

Because the limit is low enough, we can pre-calculate ~freq_x~ and ~freq_y~ for each ~x~ and ~y~ in two arrays. The time complexity of the algorithm is ~\mathcal O(N+K)~, where ~K~ is the limit on the coordinates.