#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef long double ld;

const int N = 2e5 + 5, B = 100;
int s[N], n, m;
ll sum[N];
ld dp[N];
int cnt[N];
pair<int, int> stk[N];
int top;
ld val(int l, int r)
{
	return sqrtl((sum[r] - sum[l]) * (r - l));
}
int calc(ld x)
{
	int i, l, r, mid;
	top = 0;
	stk[++top] = make_pair(0, 0);
	for(i = 1; i <= n; i++)
	{
		l = 1;
		r = top;
		while(l <= r)
		{
			mid = (l + r) >> 1;
			if(stk[mid].second > i)
				r = mid - 1;
			else
				l = mid + 1;
		}
		dp[i] = dp[stk[r].first] + val(stk[r].first, i) + x;
		cnt[i] = cnt[stk[r].first] + 1;
		while(top && stk[top].second >= i && dp[stk[top].first] + val(stk[top].first, stk[top].second) > dp[i] + val(i, stk[top].second))
			top--;
		l = max(stk[top].second, i);
		r = n;
		while(l <= r)
		{
			mid = (l + r) >> 1;
			if(dp[stk[top].first] + val(stk[top].first, mid) > dp[i] + val(i, mid))
				r = mid - 1;
			else
				l = mid + 1;
		}
		stk[++top] = make_pair(i, l);
	}
	return cnt[n];
}

int main()
{
	int i;
	ld l, r, mid;
	scanf("%d%d", &n, &m);
	for(i = 1; i <= n; i++)
		scanf("%d", s + i);
	sort(s + 1, s + n + 1);
	for(i = 1; i <= n; i++)
		sum[i] = sum[i - 1] + s[i];
	l = 0;
	r = 1e9;
	for(i = 0; i < B; i++)
	{
		mid = (l + r) / 2;
		if(calc(mid) <= m)
			r = mid;
		else
			l = mid;
	}
	printf("%.100Lf\n", dp[n] - m * l);
	return 0;
}