#include <algorithm>
#include <iomanip>
#include <iostream>
#include <vector>

using namespace std;

int main() {
  int n, k;
  cin >> n >> k;
  vector<int64_t> v(n);
  for(auto& x: v) cin >> x;
  sort(v.begin(), v.end());
  vector<int64_t> psum(n+1), spsum(n+1);
  for(int i = 0; i < n; i++) {
    psum[i+1] = psum[i] + v[i];
    spsum[i+1] = spsum[i] + v[i]*v[i];
  }
  double ret = 1e100;
  for(int i = 0; i + k <= n; i++) {
    double sev = (spsum[i+k]-spsum[i]);
    double evs = (psum[i+k]-psum[i]);
    evs *= evs;
    ret = min(ret, sev - evs/k);
  }
  cout << fixed << setprecision(39) << (ret) << "\n";
}