// need
#include <iostream>
#include <algorithm>

// data structure
#include <bitset>
//#include <list>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
//#include <array>
//#include <tuple>
#include <unordered_map>
#include <unordered_set>
//#include <complex>
//#include <deque>

// stream
//#include <istream>
//#include <sstream>
//#include <ostream>

// etc
#include <cassert>
#include <functional>
#include <iomanip>
//#include <typeinfo>
#include <chrono>
#include <random>
#include <cfenv>

#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int i=0; i<n; ++i){MACRO_VEC_ROW_Scan(i, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define OUT(d) std::cout<<d;
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<d;
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<d;
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=(n)-1;i>=0;--i)
#define FORLL(i, a, b) for(ll i=ll(a);i<ll(b);++i)
#define RFORLL(i, a, b) for(ll i=ll(b)-1;i>=ll(a);--i)
#define REPLL(i, n) for(ll i=0;i<ll(n);++i)
#define RREPLL(i, n) for(ll i=ll(n)-1;i>=0;--i)
#define PAIR std::pair<int, int>
#define PAIRLL std::pair<ll, ll>
#define IN(a, x, b) (a<=x && x<b)
#define SHOW(d) {std::cerr << #d << "\t:" << d << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWPAIR(p) {std::cerr << #p << "\t:(" << p.first << ",\t" << p.second << ")\n";}
#define SHOWPAIRVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr<<'('<<yyy.first<<", "<<yyy.second<<") ";}std::cerr << "\n";}}
#define SHOWPAIRVECTOR(v) {for(const auto& xxx:v){std::cerr<<'('<<xxx.first<<", "<<xxx.second<<") ";}std::cerr<<"\n";}
#define SHOWQUEUE(a) {std::queue<decltype(a.front())> tmp(a);std::cerr << #a << "\t:";for(int i=0; i<static_cast<int>(a.size()); ++i){std::cerr << tmp.front() << "\n";tmp.pop();}std::cerr << "\n";}
template<typename T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<typename T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }

#define int ll
using ll = long long;
using ull = unsigned long long;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^19
constexpr double EPS = 1e-7;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;

struct LUinfo {
	std::vector<double> value;
	std::vector<int> index;
};
// O( n^3 ), Gaussian forward elimination
LUinfo LU_decomposition(std::vector<std::vector<double>> A) {
	const int n = A.size();
	LUinfo data;
	for (int i = 0; i < n; ++i) {
		int pivot = i;
		for (int j = i + 1; j < n; ++j)
			if (std::abs(A[j][i]) > std::abs(A[pivot][i])) pivot = j;
		std::swap(A[pivot], A[i]);
		data.index.push_back(pivot);
		// if A[i][i] == 0, LU decomposition failed.
		for (int j = i + 1; j < n; ++j) {
			A[j][i] /= A[i][i];
			for (int k = i + 1; k < n; ++k)
				A[j][k] -= A[i][k] * A[j][i];
			data.value.push_back(A[j][i]);
		}
	}
	for (int i = n - 1; i >= 0; --i) {
		for (int j = i + 1; j < n; ++j)
			data.value.push_back(A[i][j]);
		data.value.push_back(A[i][i]);
	}
	return data;
}
// O( n^2 ) Gaussian backward substitution
std::vector<double> LU_backsubstitution(const LUinfo &data, std::vector<double> b) {
	const int n = b.size();
	int k = 0;
	for (int i = 0; i < n; ++i) {
		std::swap(b[data.index[i]], b[i]);
		for (int j = i + 1; j < n; ++j)
			b[j] -= b[i] * data.value[k++];
	}
	for (int i = n - 1; i >= 0; --i) {
		for (int j = i + 1; j < n; ++j)
			b[i] -= b[j] * data.value[k++];
		b[i] /= data.value[k++];
	}
	return b;
}

double dp[102];  // iから始めたときの最小(魔法未使用)
double dp2[102]; // iから始めたときの最小(魔法  使用)

signed main() {
	INIT;
	VAR(int, n, m);
	std::vector<int> c(n, 0);
	FOR(i, 1, n - 1) {
		std::cin >> c[i];
	}
	std::vector<std::vector<double>> A(n-1, std::vector<double>(n-1, 0));
	std::vector<double> B(n-1, 0);
	auto toPos = [&](int x) {
		if (x < n) return x;
		else return 2 * (n - 1) - x;
	};
	REP(i, n-1) {
		A[i][i] = m;
		FOR(j, 1, m + 1) {
			int p = toPos(i + j);
			if (p == n - 1) continue;
			--A[i][p];
		}
		B[i] = c[i] * m;
	}
	auto data = LU_decomposition(A);
	auto v = LU_backsubstitution(data, B);
	REP(i, n - 1) {
		dp[i] = v[i];
	}
	FOR(i, n - 1 - m, n) {
		dp2[i] = c[i];
	}
	RREP(i, n - m) {
		double min = 1e308;
		double sum = 0;
		FOR(j, i + 1, i + m + 1) {
			CHMIN(min, dp[j]);
			sum += dp2[j] / m;
		}
		dp2[i] = c[i] + std::min(min, sum);
	}
	/*REP(i, n) {
		FOUT(9, dp2[i]); SP;
	}BR;*/
	FOUT(16, dp2[0])BR;
	return 0;
}