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

#define int long long
#define ii pair<int,int>
#define iii tuple<int,int,int>
#define fi first
#define se second
#define endl '\n'

#define pub push_back
#define pob pop_back
#define puf push_front
#define pof pop_front
#define lb lower_bound
#define ub upper_bound

#define rep(x,start,end) for(int x=(start)-((start)>(end));x!=(end)-((start)>(end));((start)<(end)?x++:x--))
#define all(x) (x).begin(),(x).end()
#define sz(x) (int)(x).size()

mt19937 rng(chrono::system_clock::now().time_since_epoch().count());

#define dd double

typedef vector<int> vi;

typedef complex<double> C;
typedef vector<double> vd;
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 = (double *)&rt[j+k], y = (double *)&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 M=600;

vd mul(vd a,dd b){
	a.pub(0);
	rep(x,sz(a)-1,0){
		a[x+1]+=a[x]*b;
		a[x]*=1-b;
	}
	return a;
}

int n,k;
dd arr[50005];
vd dp;

signed main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin.exceptions(ios::badbit | ios::failbit);
	
	cin>>n>>k;
	rep(x,0,n) cin>>arr[x];
	
	sort(arr,arr+n);
	reverse(arr,arr+n);
	
	dp={1};
	dd ans=0;
	
	for (int b=0;b<n;b+=M){
		vd res;
		dd s=0;
		
		dd curr=0;
		
		for (int x=sz(dp)-1;x>=0;x--){
			int X=-b+2*x;
			
			if (X>=k+M) curr+=dp[x];
			else if (X>=k-M){
				res.pub(dp[x]);
				s=X;
			}
		}
		
		reverse(all(res));
		
		vd v={1};
		
		for (int x=b;x<min(n,b+M);x++){
			res=mul(res,arr[x]);
			
			// for (auto it:res) cout<<it<<" "; cout<<endl;
			
			s--;
			
			dd val=curr;
			for (int i=0;i<sz(res);i++){
				if (s+2*i>=k) val+=res[i];
			}
			// cout<<x<<" "<<val<<endl;
			ans=max(ans,val);
			
			v=mul(v,arr[x]);
		}
		
		dp=conv(dp,v);
	}
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
}