#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

const double EPS = 1.0e-10;

int elementaryMatrix(vector<vector<double> >& mat)
{
    const int n = mat.size();
    const int m = mat[0].size();
    int y = 0;
    int x = 0;
    while(y < n && x < m){
        int tmp = y;
        for(int i=y+1; i<n; ++i){
            if(abs(mat[i][x]) > abs(mat[tmp][x]))
                tmp = i;
        }
        if(abs(mat[tmp][x]) > EPS){
            mat[y].swap(mat[tmp]);
            for(int i=y+1; i<n; ++i){
                for(int j=m-1; j>=x; --j){
                    mat[i][j] -= mat[y][j] * (mat[i][x] / mat[y][x]);
                }
            }
            ++ y;
        }
        ++ x;
    }
    mat.resize(y);
    return y;
}

bool linearSystem(vector<vector<double> >& mat, vector<double>& x)
{
    int n = mat[0].size() - 1;
    if(elementaryMatrix(mat) != n || mat[n-1][n-1] == 0)
        return false;

    x.resize(n);
    for(int i=n-1; i>=0; --i){
        x[i] = mat[i][n];
        for(int j=i+1; j<n; ++j)
            x[i] -= mat[i][j] * x[j];
        x[i] /= mat[i][i];
    }
    return true;
}

const double INF = DBL_MAX / 10;

int main()
{
    int n, m;
    cin >> n >> m;
    vector<int> c(n, 0);
    for(int i=1; i<n-1; ++i)
        cin >> c[i];

    vector<vector<double> > mat(n, vector<double>(n+1, 0.0));
    for(int i=0; i<n-1; ++i){
        mat[i][i] = 1.0;
        mat[i][n] = c[i];
        for(int j=1; j<=m; ++j){
            int a = i + j;
            if(a > n - 1)
                a -= (a - (n - 1)) * 2;
            mat[i][a] -= 1.0 / m;
        }
    }
    mat[n-1][n-1] = 1.0;
    vector<double> x;
    linearSystem(mat, x);

    vector<double> dp(n, 0.0);
    for(int i=n-2; i>n-2-m; --i)
        dp[i] = c[i];
    for(int i=n-2-m; i>=0; --i){
        for(int j=1; j<=m; ++j)
            dp[i] += dp[i+j] / m;
        for(int j=1; j<=m; ++j)
            dp[i] = min(dp[i], x[i+j]);
        dp[i] += c[i];
    }
    printf("%.10f\n", dp[0]);

    return 0;
}