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#include <iostream>
#include <vector>
#include <algorithm>
#include <cassert>
#include <iomanip>
#include <cmath>
#include <complex>
#include <random>
#pragma GCC optimize("Ofast,fast-math")

using ll = long long;
using ld = double;

using namespace std;

#define debug(x) #x << " = " << x << '\n'
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef complex<ld> C;
typedef vector<ld> vd;
typedef pair<int, int> pii;
typedef vector<int> vi;

void fft(vector<C>& a) {
	int n = sz(a), L = 31 - __builtin_clz(n);
	static vector<complex<long double>> R(2, 1);
	static vector<C> rt(2, 1);  // (^ 10% faster if double)
	for (static int k = 2; k < n; k *= 2) {
		R.resize(n); rt.resize(n);
		auto x = polar(1.0L, acos(-1.0L) / k);
		rep(i,k,2*k) rt[i] = R[i] = i&1 ? R[i/2] * x : R[i/2];
	}
	vi rev(n);
	rep(i,0,n) rev[i] = (rev[i / 2] | (i & 1) << L) / 2;
	rep(i,0,n) if (i < rev[i]) swap(a[i], a[rev[i]]);
	for (int k = 1; k < n; k *= 2)
		for (int i = 0; i < n; i += 2 * k) rep(j,0,k) {
			// C z = rt[j+k] * a[i+j+k]; // (25% faster if hand-rolled)  /// include-line
			auto x = (ld *)&rt[j+k], y = (ld *)&a[i+j+k];        /// exclude-line
			C z(x[0]*y[0] - x[1]*y[1], x[0]*y[1] + x[1]*y[0]);           /// exclude-line
			a[i + j + k] = a[i + j] - z;
			a[i + j] += z;
		}
}
vd conv(const vd& a, const vd& b) {
	if (a.empty() || b.empty()) return {};
	vd res(sz(a) + sz(b) - 1);
	int L = 32 - __builtin_clz(sz(res)), n = 1 << L;
	vector<C> in(n), out(n);
	copy(all(a), begin(in));
	rep(i,0,sz(b)) in[i].imag(b[i]);
	fft(in);
	for (C& x : in) x *= x;
	rep(i,0,n) out[i] = in[-i & (n - 1)] - conj(in[i]);
	fft(out);
	rep(i,0,sz(res)) res[i] = imag(out[i]) / (4 * n);
	return res;
}

const int NMAX = 5e4;

ld a[NMAX + 1];

ld nw[NMAX + 1];
ld old[NMAX + 1];

int n, t;

std::vector<ld> divide(const std::vector<ld> &a) {
  if ((int) a.size() == 1) {
    return std::vector<ld>{1 - a[0], a[0]};
  }
  
  int mid = (int) a.size() / 2;
  std::vector<ld> lt, rt;
  for (int i = 0; i < mid; i++) {
    lt.push_back(a[i]);
  }
  std::vector<ld> st = divide(lt);
  lt.clear();
  for (int i = mid; i < (int) a.size(); i++) {
    rt.push_back(a[i]);
  }
  std::vector<ld> dr = divide(rt);
  rt.clear();

  std::vector<ld> ret = conv(st, dr);
  st.clear();
  dr.clear();
  return ret;
}

ld calc(int pref) {
  std::vector<ld> p(pref);
  for (int i = 0; i < pref; i++) {
    p[i] = a[i + 1];
  }
  std::vector<ld> prob = divide(p);
  ld ret = 0;
  for (int i = 0; i < (int) prob.size(); i++) {
    if (i - (pref - i) >= t) {
      ret += prob[i];
    }
  }
  return ret;
}

std::mt19937 rng(123);

int main() {
  #ifdef LOCAL
    // freopen("input.txt", "r", stdin);
  #endif
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(0);

  std::cin >> n >> t;
  
  for (int i = 1; i <= n; i++) {
    std::cin >> a[i];
  }

  std::sort(a + 1, a + n + 1);
  std::reverse(a + 1, a + n + 1);

  ld answer = 0;

  std::vector<int> even, odd;
  std::vector<int> toate;
  std::vector<int> idk;
  for (int i = t; i <= n; i++) {
    if (i % 2 == 0) {
      even.push_back(i);
    } else {
      odd.push_back(i);
    }
  }

  auto get = [&](std::vector<int> &cand) {
    if (cand.empty()) {
      return (ld) 0.00;
    }
    int l = 0, r = (int) cand.size() - 1;
    while (r - l > 5) {
      int mid1 = l + (r - l) / 3;
      int mid2 = r - (r - l) / 3;
      if (calc(cand[mid1]) < calc(cand[mid2])) {
        l = mid1;
      } else {
        r = mid2;
      }
    }
    ld ret = 0;
    for (int i = cand[l]; i <= cand[r]; i++) {
      ret = std::max(ret, calc(i));
    }
    return ret;
  };

  std::cout << std::fixed << std::setprecision(10) << std::max({get(even), get(odd)});
  
  return 0;
}