#include<bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3")


#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template <typename T> inline bool chmax(T &a, T b) {
  return ((a < b) ? (a = b, true) : (false));
}
template <typename T> inline bool chmin(T &a, T b) {
  return ((a > b) ? (a = b, true) : (false));
}


vector<double> gauss_jordan(const vector<vector<double>> &A,const vector<double> &b){
    int n=A.size();
    vector<vector<double>> B(n,vector<double>(n+1));
    rep(i,n){
        rep(j,n)B[i][j]=A[i][j];
    }
    rep(i,n)B[i][n]=b[i];
    rep(i,n){
        int pivot=i;//係数が0になることを防ぐ
        for(int j=i;j<n;j++){
            if(abs(B[j][i])>abs(B[pivot][i]))pivot=j;
        }
        swap(B[i],B[pivot]);
        for(int j=i+1;j<=n;j++)B[i][j]/=B[i][i];
        rep(j,n){
            if(i!=j){
                for(int k=i+1;k<=n;k++)B[j][k]-=B[j][i]*B[i][k];
            }
        }
    }
    vector<double> x(n);
    rep(i,n)x[i]=B[i][n];
    return x;
}

int main(){
  ll n,m;cin >> n >> m;
  using vd=vector<double>;
  vd c(n);
  for(ll i=1;i<n-1;i++)cin >> c[i];
  vector<vd> mat(n,vd(n));
  vd b(n);
  rep(i,n-1){
    mat[i][i]-=1;
    double sum=0;
    rep(_,m){
      ll to=i+_+1;
      if(to>n-1){
        ll f=to-(n-1);
        to-=f*2;
      }
      mat[i][to]+=1.0/m;
      sum+=c[to];
    }
    b[i]=-sum/m;
  }
  mat[n-1][n-1]=1;b[n-1]=0;
  auto f=gauss_jordan(mat,b);
  CST(10);
  //rep(i,n)cout << f[i] << endl;
  vd dp(n,INF);dp[n-1]=0;
  per(i,n-1){
    if(i+m>=n-1){
      dp[i]=0;continue;
    }
    double sum=0;
    double mx=INF;
    for(ll j=i+1;j<=i+m;j++){
      sum+=dp[j]+c[j];
      chmin(mx,f[j]+c[j]);
    }
    dp[i]=min(sum/m,mx);
  }
  cout << dp[0] << endl;
}