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

struct SlideMin {
    deque<pair<double, int>> q;
    int width;
    int t = 0;

    SlideMin(int width) : width(width) {}

    void add(double x) {
        t++;
        while (!q.empty() && t - q.front().second >= width) {
            q.pop_front();
        }
        while (!q.empty() && q.back().first > x) {
            q.pop_back();
        }
        q.emplace_back(x, t);
    }

    double minimum() {
        return q.front().first;
    }
};

int main() {
    int n, m;
    cin >> n >> m;
    const int M = n * 2 - 2;

    vector<double> c(M * 2);
    for (int i = 1; i < n - 1; i++) {
        cin >> c[i];
    }
    for (int i = 0; i < n - 2; i++) {
        c[n + i] = c[n - 2 - i];
    }
    for (int i = 0; i < M; i++) {
        c[i + M] = c[i];
    }

    const int T = 50000000 / M;

    double mm = 1.0 / m;

    vector<double> E(M * 2), EE(M * 2);
    for (int ii = 0; ii < T; ii++) {
        SlideMin q(m);
        double sumE = 0;
        double sumEE = 0;
        for (int i = 0; i < M; i++) {
            E[i + M] = E[i];
            EE[i + M] = EE[i];
            E[i] = 0;
            EE[i] = 0;
        }
        for (int i = m; i >= 1; i--) {
            sumE += E[M - 1 + i] + c[M - 1 + i];
            sumEE += EE[M - 1 + i] + c[M - 1 + i];
            q.add(EE[i] + c[i]);
        }
        for (int i = M - 1; i >= 0; i--) {
            if (i == n - 1) {
                E[i] = 0;
                EE[i] = 0;
            } else {
                E[i] += sumE * mm;
                EE[i] += sumEE * mm;
                E[i] = min(E[i], q.minimum());
            }
            q.add(EE[i] + c[i]);
            sumE += E[i] + c[i];
            sumEE += EE[i] + c[i];
            sumE -= E[i + m] + c[i + m];
            sumEE -= EE[i + m] + c[i + m];
        }
    }

    printf("%.20f\n", (double)min(E[0], EE[0]));
}