#include <bits/stdc++.h>
using namespace std;
long long mod1 = 100000004021LL;
long long mod2 = 100000004053LL;
long long base1 = 503;
long long base2 = 509;
int setBit(int n, int r) { return n | (1 << r); }
int resetBit(int n, int r) { return n & ~(1 << r); }
int checkBit(int n, int r) { return n & (1 << r); }
template <class T>
T sqr(T x) {
  return x * x;
}
template <class T>
T mod(T a, T b) {
  return (a % b) >= 0 ? (a % b) : (a % b) + b;
}
template <class T>
T bigmod(T p, T e, T M) {
  T ret = 1 % M;
  while (e > 0) {
    if (e % 2) ret = (ret * p) % M;
    p = (p * p) % M;
    e /= 2;
  }
  return ret;
}
template <class T>
T power(T x, T y) {
  T ans = 1;
  while (y > 0) {
    if (y % 2) ans *= x;
    x *= x;
    y /= 2;
  }
  return ans;
}
template <class T>
T gcd(T a, T b) {
  while (b != 0) {
    a = a % b;
    swap(a, b);
  }
  return a;
}
template <class T>
T lcm(T a, T b) {
  return a * (b / gcd(a, b));
}
template <class T>
void read(T &result) {
  bool sgn = false;
  result = 0;
  char ch;
  ch = getchar();
  while (true) {
    if (ch == '-') break;
    if (ch >= '0' && ch <= '9') break;
    ch = getchar();
  }
  if (ch == '-')
    sgn = true;
  else
    result = ch - '0';
  while (true) {
    ch = getchar();
    if (ch < '0' || ch > '9') break;
    result = result * 10 + (ch - '0');
  }
  if (sgn) result = -result;
}
template <class T>
ostream &operator<<(ostream &out, pair<T, T> p) {
  cout << "{" << p.first << ", " << p.second << "}";
  return out;
}
template <class T>
ostream &operator<<(ostream &out, vector<T> vec) {
  if (vec.size()) {
    cout << vec[0];
    for (int i = 1; i < vec.size(); i++) {
      cout << " " << vec[i];
    }
  }
  return out;
}
int n, k;
int a[200005];
int main() {
  cin >> n >> k;
  for (int i = 0; i < n; i++) read(a[i]);
  long long sum = 0;
  int l = 0, r = 0;
  long double ans = 0;
  while (l < n) {
    while (r < n && r - l < k) {
      sum += a[r++];
    }
    if (r - l == k) ans += sum;
    sum -= a[l++];
  }
  cout << fixed << setprecision(20) << ans / (n - k + 1) << endl;
}
