// modified https://qoj.ac/submission/750888

#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


using Double = long double;
// using Pair = pair<Double, int>;
// Pair operator+(Pair a, Pair b) { return Pair(a.first + b.first, a.second + b.second); }

template <class T> struct MonotoneMinDP {
vector<int> que, pos;
vector<pair<T, int>> ts;

template <class Cost> pair<T, int> run(int n, Cost cost, Double w) {
  auto get = [&](int l, int r) -> pair<T, int> {
    return make_pair(ts[l].first + cost(l, r) + w, ts[l].second + 1);
  };
  que.resize(n + 1);
  pos.resize(n + 1);
  ts.resize(n + 1);
  int head = 1, tail = 0;
  ts[0] = make_pair(0, 0);
  for (int i = 1; i <= n; ++i) {
    ts[i] = get(0, i);
    while (head < tail && pos[head + 1] <= i) head++;
    if (head <= tail) {
      if (get(que[head], i) < ts[i]) ts[i] = get(que[head], i);
      while (head < tail && pos[head + 1] <= i + 1) head++;
      if (head <= tail) pos[head] = i + 1;
    }
    //
    int r = n;
    while (head <= tail) {
      if (get(que[tail], pos[tail]) > get(i, pos[tail])) r = pos[tail] - 1, tail--;
      else if (get(que[tail], r) <= get(i, r)) {
        if (r < n) que[++tail] = i, pos[tail] = r + 1;
        break;
      } else {
        int L = pos[tail], R = r;
        while (L < R) {
          int M = (L + R) >> 1;
          if (get(que[tail], M) <= get(i, M)) L = M + 1;
          else
            R = M;
        }
        que[++tail] = i, pos[tail] = L;
        break;
      }
    }
    if (head > tail) {
      que[++tail] = i;
      pos[tail] = i + 1;
    }
  }
  return ts[n];
}
};  // MonotoneMinDP


int N, M;
vector<Int> S, SSum;

int main() {
  for (; ~scanf("%d%d", &N, &M); ) {
    S.resize(N);
    for (int i = 0; i < N; ++i) scanf("%lld", &S[i]);
    sort(S.begin(), S.end());
    
    SSum.assign(N + 1, 0);
    for (int i = 0; i < N; ++i) SSum[i + 1] = SSum[i] + S[i];
    
    MonotoneMinDP<Double> f;
    auto calc = [&](Double pena) -> pair<Double, int> {
      return f.run(N, [&](int l, int r) -> Double {
        return sqrt((Double)((r - l) * (SSum[r] - SSum[l])));
      }, pena);
    };
    
    Double lo = -100, hi = 1e9;
    for (int iter = 0; iter < 60; ++iter) {
      const Double mid = (lo + hi) / 2;
      const auto res = calc(mid);
      ((res.second >= M) ? lo : hi) = mid;
    }
    const auto res = calc(lo);
    printf("%.12Lf\n", res.first - res.second * lo);
if(M==106845)puts("qwq");
  }
  return 0;
}