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

vector<vector<long double>> reduce(vector<vector<long 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-11) {
            continue;
        }
        for (int ii = 0; ii < h; ii++) {
            if (ii != r) {
                long double q = a[ii][j] / a[r][j];
                for (int jj = 0; jj < w; jj++) {
                    a[ii][jj] -= q * a[r][jj];
                }
            }
        }
        long double q = a[r][j];
        for (int jj = 0; jj < w; jj++) {
            a[r][jj] /= q;
        }
        r++;
    }
    assert(r == h);
    return a;
}

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 = 100;

    vector<vector<long double>> a(M - 1, vector<long 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<long 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] = EE[i + M] = a[ii][M - 1];
        }
    }
    for (int ii = 0; ii < T; ii++) {
        for (int i = 0; i < M; i++) {
            E[i + M] = E[i];
        }
        for (int i = M - 1; i >= 0; i--) {
            E[i] = 0;
            long double mini = 1e100;
            for (int j = 1; j <= m; j++) {
                E[i] += (E[i + j] + c[i + j]) / m;
                mini = min(mini, EE[i + j] + c[i + j]);
            }
            E[i] = min(E[i], mini);
        }
    }
    printf("%.20f\n", (double)min(E[0], EE[0]));
}