#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair <double, int> pdi;

const int MAX_N = 200000 + 5;
const double EPS = 1e-9;

int N, M, a[MAX_N], g[MAX_N], ql[MAX_N], qr[MAX_N], pre[MAX_N];
ll sum[MAX_N];
double f[MAX_N];

inline pdi w(int i, int j) {
    return pair(sqrt((sum[j] - sum[i]) * (j - i)) + f[i], g[i] + 1);
}

inline bool cmp(const pdi &a, const pdi &b, const double &pen) {
    double x = a.first - b.first, y = pen * (a.second - b.second);
    if (abs(x + y) < EPS) return a.second < b.second;
    return x + y < 0;
}

void dp(double pen) {
    static int q[MAX_N];
    int hd = 0, tl = 0;
    ql[0] = 1; qr[0] = N; q[0] = 0;
    for (int i = 1; i <= N; i ++) {
        while (hd <= tl && qr[q[hd]] < i) hd ++;
        if (ql[q[hd]] <= i) ql[q[hd]] = i + 1;
        f[i] = w(q[hd], i).first;
        g[i] = g[q[hd]] + 1;
        pre[i] = q[hd];
        while (hd <= tl && !cmp(w(q[hd], ql[q[tl]]), w(i, ql[q[tl]]), pen)) tl --;
        if (tl < hd) {
            q[++ tl] = i;
            ql[i] = i + 1;
            qr[i] = N;
        }else if (cmp(w(q[tl], qr[q[tl]]), w(i, qr[q[tl]]), pen)) {
            if (qr[q[tl]] < N) {
                ql[i] = qr[q[tl]] + 1;
                qr[i] = N;
                q[++ tl] = i;
            }
        }else {
            int l = ql[q[tl]], r = qr[q[tl]];
            while (l < r) {
                int mid = l + r >> 1;
                if (!cmp(w(q[tl], mid), w(i, mid), pen)) r = mid;
                else l = mid + 1;
            }
            qr[q[tl]] = l - 1;
            ql[i] = l;
            qr[i] = N;
            q[++ tl] = i;
        }
    }
}

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 ++) {
        sum[i] = sum[i - 1] + a[i];
    }
    double l = 0, r = 1e9;
    while (r - l > 1e-9) {
        double mid = (l + r) / 2;
        dp(mid);
        if (g[N] <= M) r = mid;
        else l = mid;
    }
    dp(r);
    double ans = 0;
    for (int i = N; i; i = pre[i]) {
        ans += sqrt((sum[i] - sum[pre[i]]) * (i - pre[i]));
    }
    printf("%.15lf\n", ans);
    return 0;
}