#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <queue>
#include <set>
#include <map>
#include <bitset>
#include <random>
#include <ctime>
#include <chrono>
#include <numeric>
#include <iomanip>
#include <cassert>

typedef long long ll;
typedef long double lf;

// #define DEBUG 1
struct IO
{
    #define MAXSIZE (1 << 20)
    #define isdigit(x) (x >= '0' && x <= '9')
    char buf[MAXSIZE], *p1, *p2;
    char pbuf[MAXSIZE], *pp;
    #if DEBUG
    #else
    IO() : p1(buf), p2(buf), pp(pbuf) {}
    ~IO() {fwrite(pbuf, 1, pp - pbuf, stdout);}
    #endif
    #define gc() (p1 == p2 && (p2 = (p1 = buf) + fread(buf, 1, MAXSIZE, stdin), p1 == p2) ? ' ' : *p1++)
    #define blank(x) (x == ' ' || x == '\n' || x == '\r' || x == '\t')

    template <typename T>
    void Read(T &x)
    {
        #if DEBUG
        std::cin >> x;
        #else
        bool sign = 0; char ch = gc(); x = 0;
        for (; !isdigit(ch); ch = gc())
            if (ch == '-') sign = 1;
        for (; isdigit(ch); ch = gc()) x = x * 10 + (ch ^ 48);
        if (sign) x = -x;
        #endif
    }
    void Read(char *s)
    {
        #if DEBUG
        std::cin >> s;
        #else
        char ch = gc();
        for (; blank(ch); ch = gc());
        for (; !blank(ch); ch = gc()) *s++ = ch;
        *s = 0;
        #endif
    }
    void Read(char &c) {for (c = gc(); blank(c); c = gc());}

    void Push(const char &c)
    {
        #if DEBUG
        putchar(c);
        #else
        if (pp - pbuf == MAXSIZE) fwrite(pbuf, 1, MAXSIZE, stdout), pp = pbuf;
        *pp++ = c;
        #endif
    }
    template <typename T>
    void Write(T x)
    {
        if (x < 0) x = -x, Push('-');
        static T sta[35];
        int top = 0;
        do sta[top++] = x % 10, x /= 10; while (x);
        while (top) Push(sta[--top] ^ 48);
    }
    template <typename T>
    void Write(T x, char lst) {Write(x), Push(lst);}
} IO;
#define Read(x) IO.Read(x)
#define Write(x, y) IO.Write(x, y)
#define Put(x) IO.Push(x)

using namespace std;

const int MAXN = 2e5 + 10;
const lf eps = 1e-15;

int n, m, a[MAXN];
lf sum[MAXN];

inline lf Calc(int l, int r) { return sqrtl((sum[r] - sum[l - 1]) * (r - l + 1)); }

int st[MAXN], head, tail;
int lt[MAXN], rt[MAXN];
struct Data
{
    lf x; int y;
    Data(lf _x = 0, int _y = 0) : x(_x), y(_y) {}
    Data operator + (const lf u) const { return Data(x + u, y + 1); }
    bool operator < (const Data &u) const { return abs(x - u.x) < eps ? y < u.y : x < u.x; }
    bool operator > (const Data &u) const { return abs(x - u.x) < eps ? y > u.y : x > u.x; }
}f[MAXN];

inline bool Check(lf x)
{
    st[head = tail = 1] = 0;
    lt[0] = 1, rt[0] = n;
    f[0] = Data(0, 0);
    for (int i = 1; i <= n; i++)
    {
        while (head <= tail && rt[st[head]] < i) head++;
        f[i] = f[st[head]] + (Calc(st[head] + 1, i) + x);
        while (head <= tail && lt[st[tail]] > i && f[st[tail]] + Calc(st[tail] + 1, lt[st[tail]]) > f[i] + Calc(i + 1, lt[st[tail]])) tail--;
        if (head > tail) st[++tail] = i, lt[i] = i + 1, rt[i] = n;
        else if (f[st[tail]] + Calc(st[tail] + 1, rt[st[tail]]) > f[i] + Calc(i + 1, rt[st[tail]]))
        {
            int l = max(i + 1, lt[st[tail]]), r = rt[st[tail]], mid, res = l;
            while (l <= r)
            {
                mid = l + r >> 1;
                if (f[st[tail]] + Calc(st[tail] + 1, mid) > f[i] + Calc(i + 1, mid)) r = mid - 1, res = mid;
                else l = mid + 1;
            }
            rt[st[tail]] = res - 1;
            st[++tail] = i, lt[i] = res, rt[i] = n;
        }
        else lt[i] = rt[st[tail]] + 1, rt[i] = n, st[++tail] = i;
    }
    return f[n].y <= m;
}

int main()
{
    // freopen("F.in", "r", stdin);
    // freopen("F.out", "w", stdout);
    #if DEBUG
    #else
    ios::sync_with_stdio(0), cin.tie(0);
    #endif
    Read(n), Read(m);
    for (int i = 1; i <= n; i++) Read(a[i]);
    sort(a + 1, a + n + 1);
    for (int i = 1; i <= n; i++) sum[i] = sum[i - 1] + a[i];
    lf l = 0, r = 1e18, mid, ans = 0;
    while (r - l > eps)
    {
        mid = (l + r) / 2;
        if (Check(mid)) ans = mid, r = mid;
        else l = mid;
    }
    Check(ans);
    cout << fixed << setprecision(15);
    cout << f[n].x - ans * m << '\n';
    return 0;
}