// 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")


template <class T> struct MonotoneMinDP {
  vector<pair<T, int>> dp;
  vector<pair<int, int>> que;
  template <class Cost> pair<T, int> operator()(int n, Cost cost) {
    auto get = [&](int l, int r) -> pair<T, int> {
// assert(0<=l);assert(l<r);assert(r<=n);
// assert(0<=l);assert(l<=n);assert(0<=r);assert(r<=n);
      return make_pair(dp[l].first + cost(l, r), dp[l].second + 1);
    };
    dp.resize(n + 1);
    que.resize(n + 1);
    /*
    int head = 0, tail = 0;
    dp[0] = make_pair(T(), 0);
    for (int i = 1; i <= n; ++i) {
      dp[i] = get(0, i);
      while (head < (tail-1) && que[head + 1].second <= i) head++;
      if (head <= (tail-1)) {
        if (get(que[head].first, i) < dp[i]) dp[i] = get(que[head].first, i);
        while (head < (tail-1) && que[head + 1].second <= i + 1) head++;
        if (head <= (tail-1)) que[head].second = i + 1;
      }
      int r = n;
      while (head <= (tail-1)) {
        if (get(que[(tail-1)].first, que[(tail-1)].second) > get(i, que[(tail-1)].second)) r = que[(tail-1)].second - 1, --tail;
        else if (get(que[(tail-1)].first, r) <= get(i, r)) {
          if (r < n) que[tail++] = make_pair(i, r + 1);
          break;
        } else {
          int L = que[(tail-1)].second, R = r;
          while (L < R) {
            int M = (L + R) >> 1;
            if (get(que[(tail-1)].first, M) <= get(i, M)) L = M + 1;
            else
              R = M;
          }
          que[tail++] = make_pair(i, L);
          break;
        }
      }
      if (head > (tail-1)) {
        que[tail++] = make_pair(i, i + 1);
      }
    }
    */
    int head = 0, tail = 1;
    que[0] = make_pair(0, 1);
    for (int i = 1; i <= n; ++i) {
      for (; head + 1 < tail && que[head + 1].second <= i; ++head) {}
      dp[i] = get(que[head].first, i);
      for (; head + 1 < tail && que[head + 1].second <= i + 1; ++head) {}
      if (head < tail) que[head + 1].second = i + 1;
      for (int rr = n + 1; ; --tail) {
        if (head == tail) {
          que[tail++] = make_pair(i, i + 1);
          break;
        }
        const int l = que[tail - 1].first, r = que[tail - 1].second;
        if (get(l, r) < get(i, r)) {
          int lo = r, hi = rr;
          for (; lo + 1 < hi; ) {
            const int mid = (lo + hi) / 2;
            ((get(l, mid) < get(i, mid)) ? lo : hi) = mid;
          }
          if (hi <= n) que[tail++] = make_pair(i, hi);
          break;
        } else {
          rr = r;
        }
      }
    }
    return dp[n];
  }
};  // MonotoneMinDP


using Double = long double;

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

MonotoneMinDP<Double> f;
pair<Double, int> calc(Double pena) {
  return f(N, [&](int l, int r) -> Double {
    return sqrt((Double)((r - l) * (SSum[r] - SSum[l]))) + pena;
  });
}

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];
    
    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;
}