#include "bits/stdc++.h"
using namespace std;
#define L long long;
#ifdef LOCAL
#define dbg(x) cerr << "[" << (#x) << " = " << (x) << "]\n";
#else
#define dbg(x)
#endif
// g++ -std=c++17 -O2 -DLOCAL -o a a.cpp

double cross(vector<double> a, vector<double> b) {
  return a[0] * b[1] - a[1] * b[0];
}

double area(vector<vector<double>> arr) {
  double ans = cross(arr.back(), arr[0]);
  for (int i = 0; i + 1 < arr.size(); i++) {
    ans += cross(arr[i], arr[i + 1]);
  }
  return fabs(ans) / 2;
}

int main() {
  cin.tie(0)->sync_with_stdio(0);
  int n, select; cin >> n >> select;
  int k = n - select;
  vector<double> comp(k + 1, 1);
  if (comp.size() > 1) {
    comp[1] = double(k) * (n - k - 1) * (n - k) / (double(n) * (n - 1) * (n - 2));
    for (int y = 2; y < comp.size(); y++) {
      comp[y] = comp[y - 1] / (n - y - 1) * (k - y + 1);
    }
  }
  vector<vector<double>> p(n, vector<double>(2));
  for (int i = 0; i < n; i++) {
    cin >> p[i][0] >> p[i][1];
  }
  double total = area(p);
  for (int start = 0; start < n; start++) {
    double a = 0;
    vector<vector<double>> points = {p[start], {0, 0}, {p[(start + 1) % n]}};
    for (int y = 1; y < k + 1; y++) {
      int end = start + y + 1;
      points[1] = points[2];
      points[2] = p[end % n];
      a += area(points);
      total -= a * comp[y];
    }
  }
  cout << fixed << setprecision(10) << total << '\n';
}