// Problem         : Piece of Cake (NAIPC 2019)
// Author          : Darcy Best
// Expected Result : AC -- or possibly TLE
// Complexity      : O(n^2 log n) [log(n) just for safety, see below]
 
// Compute the probability of a (directed) edge being on the
// outside of the convex polygon and add the signed area below it.
// (Using shoelace formula)
// Compute the probabilities in a sweeping fashion.

// I'm computing all the values, then adding them together in
// a "safe-ish" way by adding similar magnitude numbers together

#include <iostream>
#include <utility>
#include <vector>
#include <algorithm>
#include <iomanip>
#include <queue>
#include <functional>
using namespace std;

double area(const pair<double,double>& p,const pair<double,double>& q){
  return p.first * q.second - p.second * q.first;
}

int main(){
  int n, k; cin >> n >> k;

  vector<pair<double,double> > A(n);
  for(auto& x : A)
    cin >> x.first >> x.second;

  reverse(begin(A), end(A)); // I want counter-clockwise
  
  priority_queue<double, vector<double>, greater<double> > pq;
  for(int i=0;i<n;i++){
    // (n-2 choose k-2) / (n choose k)
    double prob = (k * (k-1)) / 1.0 / (n * (n-1));

    int left = n-2;
    for(int dist=1;left >= k-2;dist++,left--){
      pq.emplace(prob * area(A[i],A[(i+dist)%n]));
      prob *= (left-(k-2)) / 1.0 / left;
    }
  }
  
  while((int)pq.size() > 1){
    double x = pq.top(); pq.pop();
    while(pq.size() && pq.top() > x){
      x += pq.top();
      pq.pop();
    }
    if(pq.size()){
      x += pq.top();
      pq.pop();
    }
    pq.push(x);
  }

  cout << fixed << setprecision(10) << pq.top()/2 << endl;
}