#include <bits/stdc++.h>
using namespace std;

#define _p(...) (void)printf(__VA_ARGS__)
#define forr(x,arr) for(auto&& x:arr)
#define _overload3(_1,_2,_3,name,...) name
#define _rep2(i,n) _rep3(i,0,n)
#define _rep3(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload3(__VA_ARGS__,_rep3,_rep2,)(__VA_ARGS__)
#define _rrep2(i,n) _rrep3(i,0,n)
#define _rrep3(i,a,b) for(int i=int(b)-1;i>=int(a);i--)
#define rrep(...) _overload3(__VA_ARGS__,_rrep3,_rrep2,)(__VA_ARGS__)
#define all(x) (x).begin(), (x).end()
#define bit(n) (1LL<<(n))
#define sz(x) ((int)(x).size())
#define fst first
#define snd second
using ll=long long;using pii=pair<int,int>;
using vb=vector<bool>;using vs=vector<string>;
using vi=vector<int>;using vvi=vector<vi>;using vvvi=vector<vvi>;
using vl=vector<ll>;using vvl=vector<vl>;using vvvl=vector<vvl>;
using vd=vector<double>;using vvd=vector<vd>;using vvvd=vector<vvd>;
using vpii=vector<pii>;using vvpii=vector<vpii>;using vvvpii=vector<vvpii>;
template<typename T>T read(){T t;cin>>t;return t;}
template<class T,class U>ostream&operator<<(ostream&o,const pair<T,U>&p){o<<'('<<p.fst<<", "<<p.snd<<')';return o;}

double inf = 1e15;

template<class T> bool amin(T &a, const T &b) { if (b < a) { a = b; return 1; } return 0; }

void Main() {
  int n = read<int>();
  int m = read<int>();

  vi C(n);
  rep(i, 1, n-1) C[i] = read<int>();

  int T = 1000;

  vvvd memo(n, vvd(2, vd(T+1, -1)));

  function<double(int, int, int)> dfs = [&](int cur, int used, int depth) -> double {
    if (cur == n - 1) return 0;
    if (depth == T) return C[cur];
    if (memo[cur][used][depth] != -1) return memo[cur][used][depth];
    double u = inf;
    double v = 0;

    if (used == 0) {
      rep(d, 1, m + 1) {
        int nex = cur + d;
        if (nex >= n) break;
        double ret = dfs(nex, 1, depth + 1);
        u = min(u, ret);
      }
    }
    {
      rep(d, 1, m + 1) {
        int nex = cur + d;
        if (nex >= n) nex = (n - 1) * 2 - nex;
        v += dfs(nex, used, depth + 1) / m;
      }
    }
    return memo[cur][used][depth] = min(u, v) + C[cur];
  };

  double ans = dfs(0, 0, 0);

  cout << fixed << setprecision(10) << ans << endl;
}
int main() { cin.tie(nullptr); ios::sync_with_stdio(false); Main(); return 0; }