points = [[3,-1], [4, 2], [2, 1], [1, 3], [-1, 1], [-3, 2], [-2, -2]]

def area(pts):
    n = len(pts)
    s = 0
    for i in range(n):
        a = pts[i][0] * pts[(i + 1) % n][1] - pts[i][1] * pts[(i + 1) % n][0]
        print(a)
        s += a
    return s / 2

res = area(points)
print(res)

a=lambda p:sum(x[0]*y[1]-x[1]*y[0]for x,y in zip(p,p[1:]+[p[0]]))/2
print(a(points))