def _polygon_area(x, y):
    """
    Function to compute the area of a polygon by giving its coordinates in
    counter-clockwise order.
    """
    area = 0
    j = len(x)-1; 
    for i in range (0, len(x)):
        area += (x[j]+x[i]) * (y[i]-y[j]);
        j = i
    return area/2;