#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] = {sqrtl((double)sum[i] * i) - k, 0};
	auto solve = [&](auto &solve, int l, int r) -> void
	{
		if (l == r) return;
		int mid = l + r >> 1;
		solve(solve, l, mid);
		for (int i = mid + 1, j = l; i <= r; i++)
		{
			while (j < mid)
			{
				auto tmp1 = dp[j - 1];
				tmp1.second += 1;
				tmp1.first += sqrtl((double)(sum[i] - sum[j - 1]) * (i - j + 1)) - k;
				auto tmp2 = dp[j];
				tmp2.second += 1;
				tmp2.first += sqrtl((double)(sum[i] - sum[j]) * (i - j)) - k;
				if (tmp2 < tmp1) j++;
				else break;
			}
			auto tmp = dp[j - 1];
			tmp.second += 1;
			tmp.first += sqrtl((double)(sum[i] - sum[j - 1]) * (i - j + 1)) - k;
			dp[i] = min(dp[i], tmp);
		}
		solve(solve, mid + 1, r);
	};
	solve(solve, 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 = -1e9, r = 0, mid;
	int cnt = 80;
	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';
}