#include <bits/stdc++.h>
using namespace std;
const long long INFL = 1LL << 60;
const long long INF = 1 << 30;
long long MOD = 1000000007;
vector<long long> dy = {0, 1, 0, -1, 1, 1, -1, -1, 0};
vector<long long> dx = {1, 0, -1, 0, 1, -1, 1, -1, 0};
void pres(double A, long long x = 20) {
  cout << fixed << setprecision(x) << A << endl;
}
void BinarySay(long long x, long long y = 60) {
  for (long long i = 0; i < (y); i++) cout << (x >> (y - 1 - i) & 1);
  cout << endl;
}
long long cnt_bit(long long x) { return __builtin_popcountll(x); }
long long pow_long(long long x, long long k) {
  long long res = 1;
  while (k > 0) {
    if (k % 2) res *= x;
    x *= x;
    k /= 2;
  }
  return res;
}
long long pow_mod(long long x, long long k) {
  long long res = 1;
  while (k > 0) {
    if (k % 2) {
      res *= x;
      res %= MOD;
    }
    x *= x;
    x %= MOD;
    k /= 2;
  }
  return res;
}
long long inverse(long long x) { return pow_mod(x, MOD - 2); };
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long lcm(long long x, long long y) {
  long long res = x / gcd(x, y);
  res *= y;
  return res;
};
void solve() {
  long long N, K;
  cin >> N >> K;
  vector<long long> p(N);
  for (long long i = 0; i < (N); i++) cin >> p[i];
  double tmp = 0;
  double ans = 0;
  for (long long i = 0; i < (K); i++) {
    tmp += 1.0 * (p[i] + 1) / 2;
  }
  ans = tmp;
  for (long long i = 0; i < (N - K); i++) {
    tmp += 1.0 * (p[i + K] + 1) / 2;
    tmp -= 1.0 * (p[i] + 1) / 2;
    ans = max(ans, tmp);
  }
  pres(ans);
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  solve();
}
