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

long double dp[110];

int main() {
    int n, m;
    cin >> n >> m;
    if (n == 2)
    {
        printf("0\n");
        return 0;
    }
    int c[110];
    for (int i = 1; i < n - 1; ++i) {
        cin >> c[i];
    }

    for (int i = n - m - 2; i >= 0; i--)
    {
        for (int j = 1; j <= m; j++)
            dp[i] += dp[i + j] + c[i + j];

        dp[i] /= m;
    }
    printf("%.18Lf\n", dp[0]);
    return 0;
}