#include <bits/stdc++.h>

using namespace std;

typedef double db;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    mt19937 rnd(time(0));

    int n, k;
    cin >> n >> k;
    // n = 200000, k = rnd() % n + 1;
    vector<db> a(n + 1);
    for (int i = 1; i <= n; ++i) cin >> a[i];
    // for (int i = 1; i <= n; ++i) a[i] = rnd() % 100000 + 1;
    sort(a.begin(), a.end());
    for (int i = 1; i <= n; ++i) a[i] += a[i - 1];

    auto solve = [&](db C) {
        vector<pair<db, int>> dp(n + 1, {1e20L, 0});
        vector<int> x(n + 1, 0);
        dp[0] = {0.0, 0};

        auto check = [&](int j, int i) {
            if (j >= i) return;
            db cost = sqrt((i - j) * (a[i] - a[j])) + C; // totally monotone
            // cerr << j << " " << i << " " << sqrtl((i - j) * (a[i] - a[j])) << " " << cost << " " << C << endl;
            if (dp[j].first + cost < dp[i].first) {
                dp[i] = {dp[j].first + cost, dp[j].second + 1};
                x[i] = j;
            }
        };
        auto dfs = [&](auto &&self, int l, int r) -> void {
            if (l + 1 >= r) return;
            int m = (l + r) / 2;
            for (int i = x[l]; i <= x[r]; i++) check(i, m);
            self(self, l, m);
            for (int i = l + 1; i <= m; i++) check(i, r);
            self(self, m, r);
        };
        check(0, n);
        dfs(dfs, 0, n);
        return dp[n];
    };

    db L = 0, R = 1e12;
    for (int i = 90; i; i--) {
        db m = (L + R) / 2.0;
        solve(m).second >= k ? L = m : R = m;
    }
    auto [x, y] = solve(L);
    cout << fixed << setprecision(13) << x - y * L << endl;
    return 0;
}