Draw it on the map. Take a closer look at the clockwise curve l in the topic. It is like this (for understanding, the curve is projected onto the xoy plane to form the curve L'): from a point on the positive axis of X, through the negative axis of X, through the positive axis of Y, and then back to the positive axis of X, so 2Pi to 0 is right. The directions under the two coordinates are the same.