import math
import numpy as np

# 定义一个类来表示二维点
class Point:
    def __init__(self, x=0.0, y=0.0):
        self.x = np.float128(x)
        self.y = np.float128(y)

    def input(self):
        x2, y2 = map(np.float128, input().split())
        self.x = x2
        self.y = y2

def cross(s, t):
    return s.x * t.y - s.y * t.x

maxn = 2550
# 计算组合数 C(n, m)
C = [[np.float128(0)] * (_ + 1) for _ in range(maxn)]
for i in range(0, maxn):
    for j in range(0 , i + 1):
        if j == 0 or j == i:
            C[i][j] = np.float128(1)
            continue
        if j <= i - 1:
            C[i][j] = C[i - 1][j]
        if j > 0:
            C[i][j] += C[i - 1][j - 1]

def solve():
    # 读取 n 和 k
    n, k = map(int, input().split())
    p = []

    # 输入所有的点
    for i in range(n):
        point = Point()
        point.input()
        p.append(point)

    p.reverse()  # 反转点列表

    ans = np.float128(0.0)
    for i in range(n):
        for j in range(i + 1, n):
            c = cross(p[i], p[j])
            cnt = n - (j - i + 1)
            if cnt >= k - 2:
                ans += c * C[cnt][k - 2]
            
            cnt = j - i - 1
            if cnt >= k - 2:
                ans -= c * C[cnt][k - 2]

    # 输出最终结果，使用 8 位小数
    result = ans / 2 / C[n][k]
    print(f"{result:.8f}")

if __name__ == "__main__":
    T = 1
    # T = int(input())  # 可选输入多个测试案例
    while T > 0:
        solve()
        T -= 1