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

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

    vector<long double> c(n * 2 - 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];
    }

    const int T = 20000000 / (n * m);

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