#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
using ld = long double;
constexpr int maxn = 200000 + 10;
i64 a[maxn];
ld f[maxn];
int g[maxn];
ld get(int l, int r)
{
    return f[l] + sqrtl((r - l) * (a[r] - a[l]));
}
int s[maxn], p[maxn];
int n, m;
ld calc(int x, int y)
{
    int l = y, r = n;
    while (l < r)
    {
        int m = (l + r + 1) >> 1;
        if (get(x, m) <= get(y, m)) l = m;
        else r = m - 1;
    }
    return r;
}
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cin >> n >> m;
    for (int i=1;i<=n;++i) cin >> a[i];
    sort(a + 1, a + n + 1);
    for (int i=1;i<=n;++i) a[i] += a[i - 1];
    ld L = -1e15, R = 1e15;
    int c = 100;
    while (c--)
    {
        ld M = (L + R) / 2;
        f[0] = 0;
        int l = 0, r = 0;
        for (int i=1;i<=n;++i)
        {
            while (l < r && p[l] < i) ++l;
            f[i] = get(s[l], i) + M;
            g[i] = g[s[l]] + 1;
            while (l < r)
            {
                ld x = calc(s[r], i);
                if (x <= p[r - 1]) --r;
                else break;
            }
            p[r] = calc(s[r], i);
            s[++r] = i;
        }
        if (g[n] > m) L = M;
        else R = M;
        if (!c) return cout << fixed << setprecision(15) << f[n] - M * g[n] << '\n', 0;
    }
    return 0;
}