#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cfloat>
#include <climits>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
#include <random>
using namespace std;

#define rep(i, N) for (int i = 0; i < N; i++)
#define pb push_back

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };

ll MOD = 1000000007;
ll _MOD = 1000000009;
int INF = INT_MAX / 2;
double EPS = 1e-10;

int main() {
	int N, M; cin >> N >> M;
	vector<int> a(N);
	for (int i = 1; i <= N - 2; i++)
		cin >> a[i];
	vector<vector<double> > dp(N, vector<double>(2));
	rep(t, 1000) {
		vector<vector<double> > _dp(N, vector<double>(2));
		for (int i = 0; i <= N - 2; i++) {
			double sum0 = 0, sum1 = 0, mi = DBL_MAX;
			for (int j = 1; j <= M; j++) {
				int k = i + j;
				if (k >= N) k = (N - 2) - (k - N);
				sum0 += dp[k][0] + a[k];
				sum1 += dp[k][1] + a[k];
				mi = min(mi, dp[k][0] + a[k]);
			}
			dp[i][0] = sum0 / M;
			dp[i][1] = min(sum1 / M, mi);
		}
	}
	printf("%.15f\n", dp[0][1]);
}