#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if (y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if (x < y) x = y; }

typedef double Num;
typedef vector<Num> Vec;
typedef vector<Vec> Mat;

Vec gaussian_elimination(Mat A, Vec b) {
	const int n = A.size(), m = A[0].size();
	int pi = 0, pj = 0;
	while (pi < n && pj < m) {
		reu(i, pi + 1, n)
			if (abs(A[i][pj]) > abs(A[pi][pj]))
				A[i].swap(A[pi]), swap(b[i], b[pi]);
		if (abs(A[pi][pj]) > 0) {
			Num d = A[pi][pj];
			rep(j, m) A[pi][j] /= d;
			b[pi] /= d;
			reu(i, pi + 1, n) {
				Num k = A[i][pj];
				rep(j, m) A[i][j] -= k * A[pi][j];
				b[i] -= k * b[pi];
			}
			pi++;
		}
		pj++;
	}
	//reu(i, pi, n) if (abs(b[i]) > 0) return Vec();
	//if (pi < m || pj < m) return Vec();
	for (int j = m - 1; j >= 0; j--)
		rep(i, j)
		b[i] -= b[j] * A[i][j];
	return b;
}

int main() {
	int n; int m;
	while (~scanf("%d%d", &n, &m)) {
		vector<int> c(n);
		for (int i = 1; i < n - 1; ++ i)
			scanf("%d", &c[i]);
		Mat A(m, Vec(m));
		Vec b(m);
		rep(i, m) {
			A[i][i] = -1;
			if (i != 0) {
				rer(k, 1, m)
					A[i][abs(i - k)] += 1. / m;
			}
			b[i] = -c[n - 1 - i];
		}
		Vec x = gaussian_elimination(A, b);
		double dp[100][2] = {};
		rep(i, m) {
			dp[n - 1 - i][0] = x[i];
			dp[n - 1 - i][1] = c[n - 1 - i];
		}
		for (int i = n - 1 - m; i >= 0; --i) {
			rep(j, 2) {
				double x = 0;
				rer(k, 1, m)
					x += dp[i + k][j];
				x /= m;
				if (j > 0) {
					rer(k, 1, m)
						amin(x, dp[i + k][j - 1]);
				}
				dp[i][j] = c[i] + x;
			}
		}
		double ans = dp[0][1];
		printf("%.10f\n", ans);
	}
	return 0;
}