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

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 = 40000000 / (n * m);

    vector<double> E(M * 2), EE(M * 2);
    for (int ii = 0; ii < T; ii++) {
        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 - 1; i >= 0; i--) {
            if (i == n - 1) {
                E[i] = 0;
                EE[i] = 0;
            } else {
                double mini = 1e100;
                for (int j = 1; j <= m; j++) {
                    double d = c[i + j];
                    E[i] += (E[i + j] + d) / m;
                    EE[i] += (EE[i + j] + d) / m;
                    mini = min(mini, EE[i + j] + d);
                }
                E[i] = min(E[i], mini);
            }
        }
    }

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