#include <bits/stdc++.h>
#define ll long long
#define ldb long double
using namespace std;
const ldb eps = 1e-9;
int n, m, a[200010];
ldb D, f[200010];
struct Dat 
{
	int x, k;
} q[200010]; 
int hh, tt;
ldb val(int j, int i) 
{
	return sqrtl(1.0 * (a[i] - a[j]) * (i - j));
}

bool cmp(int i, int j, int k) 
{
	return f[i] + val(i, k) <= f[j] + val(j, k);
}

ldb DP() 
{
	q[0] = (Dat){1, 0}, hh = tt = 0;
	for(int i = 1; i <= n; i ++)
	{
		while (hh < tt && q[hh + 1].x <= i) hh ++;
		f[i] = f[q[hh].k] + val(q[hh].k, i) + D;
		int l = i + 1, r = n + 1;
		while (hh < tt && cmp(i, q[tt].k, q[tt].x)) r = q[tt --].x;
		if (l > q[tt].x) q[tt].x = l;
		else l = q[tt].x + 1;
		while(l < r)
		{
			int mid = l + r >> 1;
			if(cmp(i, q[tt].k, mid)) r = mid;
			else l = mid + 1;
		}
		q[++ tt] = (Dat){r, i};
	}
	return f[n] - m * D;
}

int main()
{
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; i ++)
		scanf("%d", a + i);
	sort(a + 1, a + n + 1);
	for(int i = 1; i <= n; i ++)
		a[i] += a[i - 1];
	ldb l = 0, r = sqrtl(1.0 * n * a[n]), md, xl, xr, vl, vr;
	while (r - l > eps) 
	{
		md = (l + r) / 2.0;
		xl = D = md - eps / 6.0, vl = DP();
		xr = D = md + eps / 6.0, vr = DP();
		if (vl > vr) r = xr;
		else l = xl;
	}
	D = (l + r) / 2;
	printf("%.30Lf\n", DP());
	return 0;
}