# include <bits/stdc++.h>
# define FILE(x) freopen(x".in", "r", stdin); freopen(x".out", "w", stdout);
using namespace std;
const int N = 105;
int n;
double m;
double a[N];
double f[N][2];
int main()
{   
    //FILE("turnback");
    cin >> n >> m;
    for (int i = 2; i < n; i ++)
    {
        cin >> a[i];
    }
    for (int i = n - 1; i >= 1; i --)
    {
        double sum = 0;
        for (int k = 1; k <= m; k ++)
        {
            int j = i + k;
            if (j > n) j = 2 * n - j;
            sum += f[j][1];
        }
        f[i][1] = a[i] + sum * 1.0 / m;
        sum = 0;
        for (int k = 1; k <= m; k ++)
        {
            int j = i + k;
            if (j > n) j = 2 * n - j;
            sum += f[j][0];
        }
        double op = a[i] + sum / m;
        double res = 1e18;
        for (int k = 1; k <= m; k ++)
        {
            int j = i + k;
            if (j > n) j = 2 * n - j;
            res = min(res, f[j][1]);
        }
        double op2 = a[i] + res;
        f[i][0] = min(op, op2);
    }
    printf("%.9lf", f[1][0]);
    return 0;
}