#include <iostream>
#include <sstream>
#include <iomanip>
#include <cstdio>
#include <cassert>
#include <algorithm>
#include <iterator>
#include <string>
#include <vector>
#include <list>
#include <deque>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <tuple>
#include <queue>
#include <stack>
#include <functional>
#include <utility>
#include <complex>
#include <bitset>
#include <numeric>
#include <random>
using namespace std;

#define REP(i,n) for(int (i)=0; (i)<(n) ;++(i))
#define REPN(i,a,n) FOR((i),(a),(a)+(n))
#define FOR(i,a,b) for(int (i)=(a); (i)<(b) ;++(i))
#define PB push_back
#define MP make_pair
#define SE second
#define FI first
#define DBG(a) cerr<<(a)<<endl;
#define dump(a) cerr<<#a <<' '<< a <<endl;
#define ALL(v) (v).begin(),(v).end()
typedef long long LL;  typedef pair<LL, LL> PLL; typedef vector<LL> VLL;
typedef pair<int,int>PI; typedef vector<int> VI;
const LL LINF=334ll<<53; const int INF=15<<26; const LL MOD=1E9+7;
const double eps=1e-12;
typedef vector<vector<double >> Matrix;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout <<fixed<< setprecision(11);
    int n,m;
    cin >> n >> m;
    vector<double> p(n),c(n);
    FOR(i,1,n-1){
        cin >> c[i];
    }
    for(int i=n-2-m;i>=0;--i){
        FOR(j,i+1,i+m+1){
            p[i]+=p[j]+c[j];
        }
        p[i]/=m;
    }
    cout << p[0]<<endl;
    return 0;
}