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

template<class T> ostream &operator <<(ostream &os, const vector<T> &v) {
    for (T x : v) os << x << " ";
    return os;
}

vector<vector<double>> reduce(vector<vector<double>> a) {
    int h = a.size();
    int w = a[0].size();
    int r = 0;
    for (int j = 0; j < w && r < h; j++) {
        int p = r;
        for (int i = r + 1; i < h; i++) {
            if (abs(a[i][j]) > abs(a[p][j])) {
                p = i;
            }
        }
        swap(a[p], a[r]);
        if (abs(a[r][j]) < 1e-8) {
            continue;
        }
        for (int ii = 0; ii < h; ii++) {
            if (ii != r) {
                double q = a[ii][j] / a[r][j];
                for (int jj = 0; jj < w; jj++) {
                    a[ii][jj] -= q * a[r][jj];
                }
            }
        }
        double q = a[r][j];
        for (int jj = 0; jj < w; jj++) {
            a[r][jj] /= q;
        }
        r++;
    }
    return a;
}

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 = 30000000 / M;

    double mm = 1.0 / m;

    vector<vector<double>> a(M - 1, vector<double>(M));
    for (int i = 0; i < M; i++) {
        if (i == n - 1) {
            continue;
        }
        int ii = i > n - 1 ? i - 1 : i;
        a[ii][ii] = 1;

        for (int j = 1; j <= m; j++) {
            int jj = (i + j) % M;
            if (jj != n - 1) {
                int jjj = jj > n - 1 ? jj - 1 : jj;
                a[ii][jjj] -= 1.0 / m;
                a[ii][M - 1] += c[jj] / m;
            } else {
                a[ii][M - 1] += c[jj] / m;
            }
        }
    }
    a = reduce(a);
    vector<double> E(M * 2), EE(M * 2);
    for (int i = 0; i < M; i++) {
        if (i != n - 1) {
            int ii = i > n - 1 ? i - 1 : i;
            E[i] = EE[i] = a[ii][M - 1];
        }
    }
    SlideMin q(m);
    for (int i = m - 1; i >= 0; i--) {
        q.add(EE[i] + c[i]);
    }
    for (int ii = 0; ii < T; ii++) {
        double sumE = 0;
        double sumEE = 0;
        for (int i = 0; i < M; i++) {
            E[i + M] = E[i];
            EE[i + M] = EE[i];
        }
        for (int i = 0; i < m; i++) {
            sumE += E[i + M] + c[i + M];
            sumEE += EE[i + M] + c[i + M];
        }
        for (int i = M - 1; i >= 0; i--) {
            E[i] = 0;
            EE[i] = 0;
            if (i != n - 1) {
                E[i] = min(sumE * mm, q.minimum());
                EE[i] += sumEE * mm;
            }
            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]));
}