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

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++;
    }
    assert(r == h);
    return a;
}

int main() {
    int n, m;
    cin >> n >> m;

    vector<double> c(n);
    for (int i = 1; i < n - 1; i++) {
        cin >> c[i];
    }
    
    vector<vector<double>> a(n - 1, vector<double>(n));

    for (int i = 0; i < n - 1; i++) {
        a[i][i] = 1;
        for (int j = 1; j <= m; j++) {
            int ii = i + j;
            if (ii >= n - 1) {
                ii = 2 * (n - 1) - ii;
            }
            if (ii < n - 1) {
                a[i][ii] -= 1.0 / m;
            }
            a[i][n - 1] += c[ii] / m;
        }
    }

    a = reduce(a);

    vector<double> dp(n);
    for (int i = n - 2 - m; i >= 0; i--) {
        double mini = 1e100;
        for (int j = 1; j <= m; j++) {
            dp[i] += (dp[i + j] + c[i + j]) / m;
            mini = min(mini, a[i + j][n - 1] + c[i + j]);
        }
        dp[i] = min(dp[i], mini);
    }

    printf("%.20f\n", dp[0]);
}