def cross(a, b):
    return a[0] * b[1] - a[1] * b[0]

def area(arr):
    ans = cross(arr[-1], arr[0])
    for a, b in zip(arr, arr[1:]):
        ans += cross(a, b)
    return abs(ans) / 2


n, select = map(int, input().split())
k = n - select

comp = [1 for _ in range(k+1)]

if len(comp) > 1:
    comp[1] = k * (n - k - 1) * (n - k) / (n * (n - 1) * (n - 2))
    for y in range(2, len(comp)):
        comp[y] = comp[y-1] / (n - y - 1) * (k - y + 1)


p = [list(map(float, input().split())) for _ in range(n)]


total = area(p)
for start in range(n):
    a = 0
    points = [p[start], [0, 0], p[(start + 1) % n]]
    for y in range(1, k+1):
        end = start + y + 1
        points[1] = points[2]
        points[2] = p[end%n]
        a += area(points)
        total -= a * comp[y]
print(total)