#include "bits/stdc++.h"


#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep1(i, n) for (int i = 1; i < (n); ++i)
#define rep1n(i, n) for (int i = 1; i <= (n); ++i)
#define repr(i, n) for (int i = (n) - 1; i >= 0; --i)
#define pb push_back
#define eb emplace_back
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define each(x, a) for (auto &x : a)
#define ar array
#define vec vector
#define range(i, n) rep(i, n)

using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using str = string;
using pi = pair<int, int>;
using pl = pair<ll, ll>;

using vi = vector<int>;
using vl = vector<ll>;
using vpi = vector<pair<int, int>>;
using vvi = vector<vi>;

int Bit(int mask, int b) { return (mask >> b) & 1; }

template<class T>
bool ckmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool ckmax(T &a, const T &b) {
    if (b > a) {
        a = b;
        return true;
    }
    return false;
}

// [l, r)
template<typename T, typename F>
T FindFirstTrue(T l, T r, const F &predicat) {
    --l;
    while (r - l > 1) {
        T mid = l + (r - l) / 2;
        if (predicat(mid)) {
            r = mid;
        } else {
            l = mid;
        }
    }
    return r;
}


template<typename T, typename F>
T FindLastFalse(T l, T r, const F &predicat) {
    return FindFirstTrue(l, r, predicat) - 1;
}

const int INFi = 2e9;
const ll INF = 2e18;
const int LG = 20;

double start;
double getTime() {
    return ((double) clock() - start) / CLOCKS_PER_SEC;
}

void solve() {
    start = clock();

    int n, m;
    cin >> n >> m;
    vector<ll> s(n);
    for (auto &x : s) {
        cin >> x;
    }
    sort(all(s));

    vector<ll> psum(n + 1);
    for (int i = 0; i < n; i++) {
        psum[i + 1] = psum[i] + s[i];
    }

    auto solve = [&] (ld lambda) {
        vector<pair<ld, int>> dp(n + 1, {INF, INFi});
        dp[0] = {0, 0};
        vector<pair<int, int>> stk;
        stk.emplace_back(0, 0); // (from, who)

        auto calc = [&] (int i, int j) {
            auto val = dp[i];
            val.first += sqrtl((psum[j] - psum[i]) * (j - i)) + lambda;
            val.second++;
            return val;
        };
        for(int i = 1; i <= n; ++i) {
            {
                int pos = lower_bound(all(stk), make_pair(i + 1, -1)) - stk.begin();
                pos--;
                dp[i] = calc(stk[pos].second, i);
            }
            int r = n + 1;
            while (!stk.empty() && r > i + 1) {
                auto [from, who] = stk.back();
                if (from > i && calc(who, from) > calc(i, from)) {
                    r = from;
                    stk.pop_back();
                    continue;
                }

                int lq = max(from, i);
                int rq = r;
                while(rq - lq > 1) {
                    int mid = (lq + rq) / 2;
                    if (calc(who, mid) < calc(i, mid)) {
                        lq = mid;
                    } else {
                        rq = mid;
                    }
                }
                r = rq;
                break;
            }
            if (r != n + 1) {
                stk.emplace_back(r, i);
            }
        }
        return dp[n];
    };

    ld L = -1e18;
    ld R = 1e18;
    while (getTime() < 5.5) {
        ld M = (L + R) / 2;
        if (solve(M).second > m) {
            L = M;
        } else {
            R = M;
        }
    }
    auto [val, k] = solve(L);
    cout << val - m * L << '\n';
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(12) << fixed;
    int t = 1;
//    cin >> t;
    rep(i, t) {
        solve();
    }
    return 0;
}