#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ld = long double;

#define all(a) begin(a), end(a)
#define len(a) int((a).size())

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);
    cout << fixed << setprecision(15);

    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];
    }

    vector<pair<int, int>> segs(m);
    for (int i = 0; i < m; i++) {
        segs[i] = {i, i};
    }
    segs.back().second = n - 1;

    auto eval = [&](pair<int, int> seg) {
        return sqrtl((psum[seg.second + 1] - psum[seg.first]) * (seg.second - seg.first + 1));
    };

    priority_queue<tuple<ld, int, int>> pq;

    auto get = [&](int i, int d) -> ld {
        ld now = eval(segs[i]) + eval(segs[i + 1]);
        if (segs[i].second + d >= segs[i].first && segs[i + 1].first + d <= segs[i + 1].second) {
            ld then = eval({segs[i].first, segs[i].second + d}) + eval({segs[i + 1].first + d, segs[i + 1].second});
            if (now - then < 1e-9) {
                return -1e18;
            }
            return now - then;
        }
        return -1e18;
    };

    auto push = [&](int i) {
        if (i < 0 || i + 1 >= m) {
            return;
        }
        pq.emplace(get(i, -1), i, -1);
        pq.emplace(get(i, 1), i, 1);
    };

    for (int i = 0; i + 1 < m; i++) {
        push(i);
    }

    while (!pq.empty()) {
        auto [delta, i, d] = pq.top();
        pq.pop();
        if (delta < 1e-9) {
            break;
        }
        if (fabs(delta - get(i, d)) > 1e-8) {
            continue;
        }

        segs[i].second += d;
        segs[i + 1].first += d;
        push(i - 1);
        push(i);
        push(i + 1);
    }

    ld ans = 0;
    for (auto s : segs) {
        ans += eval(s);
    }
    cout << ans << '\n';
}