Assume $(x_1, y_1), (x_2, y_2), \cdots, (x_n, y_n)$ are $n$ points in the plane in the counterclockwise order, then the area enclosed by those points is $$ \frac12((x_1y_2-x_2y_1)+(x_2y_3-x_3y_2)+\cdots +(x_ny_1-x_1y_n)) $$