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//~ mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
//~ while (clock()<=69*CLOCKS_PER_SEC)
//~ #pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("O3")
//~ #pragma GCC target ("avx2")
//~ #pragma GCC optimize("Ofast")
//~ #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//~ #pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
using namespace std;

template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
  enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
  ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
  *this << "[";
  for (auto it = d.b; it != d.e; ++it)
    *this << ", " + 2 * (it == d.b) << *it;
  ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "

#define shandom_ruffle random_shuffle

using ll=long long;
using pii=pair<int,int>;
using pll=pair<ll,ll>;
using vi=vector<int>;
using vll=vector<ll>;
const int nax=1000*1007;
using ld=long double;

/* Prec. error max_ans/1e15 (2.5e18) for (long) doubles, so int rounding works
for doubles with answers 0.5e15, e.g. for sizes 2^20 and RANDOM ints in [0,45k],
assuming DBL_MANT_DIG=53 and LDBL_MANT_DIG=64. Consider normalizing and brute.*/
#define REP(i,n) for(int i = 0; i < int(n); ++i)
struct C { ld re, im;
	C operator * (const C & he) const {
		return C{re * he.re - im * he.im,
				re * he.im + im * he.re};
	}
	void operator += (const C & he) { re += he.re; im += he.im; }
};
void dft(vector<C> & a, bool rev) {
	const int n = a.size();
	for(int i = 1, k = 0; i < n; ++i) {
		for(int bit = n / 2; (k ^= bit) < bit; bit /= 2);;;
		if(i < k) swap(a[i], a[k]);
	}
	for(int len = 1, who = 0; len < n; len *= 2, ++who) {
		static vector<C> t[30];
		vector<C> & om = t[who];
		if(om.empty()) {
			om.resize(len);
			const ld ang = 2 * acosl(0) / len;
			REP(i, len) om[i] = i%2 || !who ?
					C{cos(i*ang), sin(i*ang)} : t[who-1][i/2];
		}
		for(int i = 0; i < n; i += 2 * len)
			REP(k, len) {
				 const C x = a[i+k], y = a[i+k+len]
						* C{om[k].re, om[k].im * (rev ? -1 : 1)};
				a[i+k] += y;
				a[i+k+len] = C{x.re - y.re, x.im - y.im};
			}
	}
	if(rev) REP(i, n) a[i].re /= n;
}
template<typename T>vector<T> multiply(const vector<T> & a, const vector<T> & b) {
	if(a.empty() || b.empty()) return {};
	T big = 0;
	int n = a.size() + b.size();
	vector<T> ans(n - 1);
	/* if(min(a.size(),b.size()) < 190) { // BRUTE FORCE
		REP(i, a.size()) REP(j, b.size()) ans[i+j] += a[i]*b[j];
		return ans; } */
	while(n&(n-1)) ++n;
	auto foo = [&](const vector<C> & w, int i, int k) {
		int j = i ? n - i : 0, r = k ? -1 : 1;
		return C{w[i].re + w[j].re * r, w[i].im
				- w[j].im * r} * (k ? C{0, -0.5} : C{0.5, 0});
	};
	vector<C> in(n), done(n);
	REP(i, a.size()) in[i].re = a[i] - big;
	REP(i, b.size()) in[i].im = b[i] - big;
	dft(in, false);
	REP(i, n) done[i] = foo(in, i, 0) * foo(in, i, 1);
	dft(done, true);
	REP(i, ans.size()) ans[i] = is_integral<T>::value ?
			llround(done[i].re) : done[i].re;
	return ans;
}

int n, k;

ld tab[nax];

ld pref[nax];

ld czyt()
{
	double x;
	scanf("%lf", &x);
	return x;
}

int decyzja(int a, int b, int v)//-1 to przegrana, 1 to wygrana, 0 nie wiadomo
{
	int p=v-(a-v);
	int dolne=((p-(b-a))>=k);
	int gorne=((p+(b-a))>=k);
	if (gorne==0)
		return -1;
	if (dolne==1)
		return 1;
	return 0;
}

vector<ld> drz[nax];//przedzial bazowy [a, b] trzyma iloczyn dla [a+1, b+1]

void prepro(int v, int a, int b)
{
	if (a==b)
	{
		drz[v]={1-tab[a+1], tab[a+1]};
		return;
	}
	
	int s=(a+b+2)>>1;
	prepro(v*2, a, s-1);
	prepro(v*2+1, s, b);
	
	int r1=drz[v*2].size();
	int r2=drz[v*2+1].size();
	int r=r1+r2-1;
	drz[v]=multiply(drz[v*2], drz[v*2+1]);
	drz[v].resize(r);
}


void rek(int v, int a, int b, vector<ld> w, int dol, int gor)
{
	assert((int)w.size()-1==gor-dol);
	{
		int d=dol;
		int g=gor;
		while(d!=g && decyzja(a, b, d+1)==-1)
			d++;
		while(d!=g && decyzja(a, b, g-1)==1)
			g--;
		vector<ld> now(g-d+1);
		for (int i=dol; i<=gor; i++)
			now[min(max(i, d), g)-d]+=w[i-dol];
		
		w=now;
		dol=d;
		gor=g;
	}
	assert((int)w.size()-1==gor-dol);
	
	if (a==b)
	{
		for (int i=dol; i<=gor; i++)
			if (i-(a-i)>=k)
				pref[a]+=w[i-dol];
		return;
	}
	
	int s=(a+b+2)>>1;
	rek(v*2, a, s-1, w, dol, gor);
	
	vector<ld> now=multiply(w, drz[v*2]);
	int roz=drz[v*2].size();
	now.resize(gor-dol+roz);
	rek(v*2+1, s, b, now, dol, gor+roz-1);
}

int main()
{
	scanf("%d%d", &n, &k);
	for (int i=1; i<=n; i++)
		tab[i]=czyt();
	sort(tab+1, tab+1+n);
	reverse(tab+1, tab+1+n);
	
	prepro(1, 0, n);
	rek(1, 0, n, {1}, 0, 0);
	
	ld wyn=0;
	for (int i=1; i<=n; i++)
		wyn=max(wyn, pref[i]);
	printf("%.15lf\n", (double)wyn);
	return 0;
}