#include <stdio.h>

#include <cmath>
#include <vector>

#define LOG(FMT...) fprintf(stderr, FMT)

using namespace std;

typedef double DB;
typedef long long LL;
constexpr int N = 200005;
constexpr DB eps = 1e-9;

int n, m;
LL a[N];
DB D, f[N];

struct Dat {
	int x, k;
} q[N]; int hh, tt;

DB val(int j, int i) {
	return sqrt((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);
}

DB 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, md;
		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) {
			md = l + r >> 1;
			if (cmp(i, q[tt].k, md)) {
				r = md;
			} else {
				l = md + 1;
			}
		}
		if (r <= n) {
			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("%lld", a + i);
		a[i] += a[i - 1];
	}
	DB l = 0, r = sqrt(n * a[n]), xl, xr, vl, vr;
	while (r - l > eps) {
		xl = D = (2 * l + r) / 3, vl = DP();
		xr = D = (l + 2 * r) / 3, vr = DP();
		if (vl > vr) {
			r = xr;
		} else {
			l = xl;
		}
	}
	D = (l + r) / 2;
	printf("%.30lf\n", DP());
	return 0;
}