#include <iostream>
#include <cstdio>
#include <fstream>
#include <algorithm>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <vector>
#include <limits.h>
#include <math.h>
#include <functional>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <float.h>
#include <random>

#define repeat(i,n) for (long long i = 0; (i) < (n); ++ (i))
#define debug(x) cerr << #x << ": " << x << '\n'
#define debugArray(x,n) for(long long hoge = 0; (hoge) < (n); ++ (hoge)) cerr << #x << "[" << hoge << "]: " << x[hoge] << '\n'
#define debugArrayP(x,n) for(long long hoge = 0; (hoge) < (n); ++ (hoge)) cerr << #x << "[" << hoge << "]: " << x[hoge].first<< " " << x[hoge].second << '\n'

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> Pii;
typedef vector<int> vint;
typedef vector<ll> vll;
const double INF = DBL_MAX;
const ll MOD = 998244353;

typedef double number;
const number eps = 1e-8;
typedef vector<number> Array;
typedef vector<Array> Matrix;

struct LUinfo {
  vector<number> value;
  vector<int> index;
};
// O( n^3 ), Gaussian forward elimination
LUinfo LU_decomposition(Matrix 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 (abs(A[j][i]) > abs(A[pivot][i])) pivot = j;
    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
Array LU_backsubstitution(const LUinfo &data, Array b) {
  const int n = b.size();
  int k = 0;
  for (int i = 0; i < n; ++i){
    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;
}

int n,m;
double dp[110];

int main(){
  cin >>n>>m;n--;
  Array c(n);
  repeat(i,n-1){
    cin>>c[i+1];
  }
  Matrix A(n,Array(n,0));
  repeat(i,n){
    A[i][i] = 1;
    repeat(j,m){
      int jj=i+1+j;
      if(jj==n){
        continue;
      }
      if(jj>n){
        jj = n-(jj-n);
      }
      A[i][jj] -= 1./m;
    }
  }
  Array f = LU_backsubstitution(LU_decomposition(A),c);
  for(int i=n-1;i>=0;i--){
    dp[i] = INF;
    if(i+m>=n){
      dp[i] = c[i];
      continue;
    }
    double ave = 0;
    repeat(j,m){
      dp[i] = min(dp[i],f[i+1+j]);
      ave += dp[i+1+j];
    }
    ave /= m;
    dp[i] = min(dp[i],ave);
    dp[i] += c[i];
  }
  printf("%lf\n",dp[0]);
  return 0;
}