#include <bits/stdc++.h>
using namespace std;

const int N = 2e5 + 5;
int n, m, s[N];
long long sum[N];
#define double long double
pair<double, int> dp[N];
pair<double, int> check(double k)
{
	for (int i = 1; i <= n; i++) dp[i] = {1e18, 0};
	auto solve = [&](auto &solve, int l, int r, int a, int b) -> void
	{
		if (l > r) return;
		if (a == b)
		{
			for (int i = l; i <= r; i++)
			{	
				auto tmp = dp[a - 1];
				tmp.second += 1;
				tmp.first += sqrtl((double)(sum[i] - sum[a - 1]) * (i - a + 1)) - k;
				dp[i] = min(dp[i], tmp);
			}
			return;
		}
		int mid = l + r >> 1, p = a;
		solve(solve, l, mid - 1, a, b);
		pair<double, int> mi{1e18, 0};
		for (int i = a; i <= b and i <= mid; i++)
		{
			auto tmp = dp[i - 1];
			tmp.second += 1;
			tmp.first += sqrtl((double)(sum[mid] - sum[i - 1]) * (mid - i + 1)) - k;
			if (tmp < mi) mi = tmp, p = i;
		}
		dp[mid] = min(dp[mid], mi);
		solve(solve, mid + 1, r, p, b);
	};
	solve(solve, 1, n, 1, n);
	return dp[n];
}

pair<double, int> check1(double k)
{
	for (int i = 1; i <= n; i++) dp[i] = {1e18, 0};
	auto solve = [&](auto &solve, int l, int r, int a, int b) -> void
	{
		if (l > r) return;
		if (a == b)
		{
			for (int i = l; i <= r; i++)
			{	
				auto tmp = dp[a - 1];
				tmp.second += 1;
				tmp.first += sqrtl((double)(sum[i] - sum[a - 1]) * (i - a + 1)) - k;
				dp[i] = min(dp[i], tmp);
			}
			return;
		}
		int mid = l + r >> 1, p = a;
		pair<double, int> mi{1e18, 0};
		for (int i = a; i <= b and i <= mid; i++)
		{
			auto tmp = dp[i - 1];
			tmp.second += 1;
			tmp.first += sqrtl((double)(sum[mid] - sum[i - 1]) * (mid - i + 1)) - k;
			if (tmp < mi) mi = tmp, p = i;
		}
		cout << l << ' ' << r << ' ' << a << ' ' << b << " : " << mid << ' ' << p << '\n';
		dp[mid] = min(dp[mid], mi);
		solve(solve, l, mid - 1, a, p);
		solve(solve, mid + 1, r, p, b);
	};
	solve(solve, 1, n, 1, n);
	return dp[n];
}

int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	
	cin >> n >> m;
	for (int i = 1; i <= n; i++) cin >> s[i];
	sort(s + 1, s + n + 1);
	for (int i = 1; i <= n; i++) sum[i] = sum[i - 1] + s[i];
	
	double l = -1e18, r = 0, mid;
	int cnt = 200;
	while (cnt--)
	{
		mid = (l + r) / 2;
		if (check(mid).second >= m) r = mid;
		else l = mid;
	}
	double ans = check(r).first + r * m; 
	cout << fixed << setprecision(10) << ans << '\n';
}