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#include <bits/stdc++.h>
using namespace std;
#define fwd(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) X.begin(), X.end()
#define sz(X) int(size(X))
#define pb push_back
#define eb emplace_back
#define st first
#define nd second
using pii = pair<int, int>; using vi = vector<int>;
using ll = long long; using ld = long double;
#ifdef LOC
auto SS = signal(6, [](int) { *(int *)0 = 0; });
#define DTP(x, y) auto operator << (auto &o, auto a) -> decltype(y, o) { o << "("; x; return o << ")"; }
DTP(o << a.st << ", " << a.nd, a.nd);
DTP(for (auto i : a) o << i << ", ", all(a));
void dump(auto... x) { (( cerr << x << ", " ), ...) << '\n'; }
#define deb(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#define deb(...) 0
#endif


// https://github.com/kth-competitive-programming/kactl/blob/main/kactl.pdf

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<ld>> R(2,1);
	static vector<C> rt(2,1);
	for(static int k = 2; k < n; k *= 2) {
		R.resize(n); rt.resize(n);
		auto x = polar(1.0L, acos(-1.0L) / k);
		fwd(i,k,2*k) rt[i] = R[i] = i&1 ? R[i/2] * x : R[i/2];
	}
	vi rev(n);
	rep(i,n) rev[i] = (rev[i/2] | (i & 1) << L) / 2;
	rep(i, 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,k) {
			C z = rt[j+k] * a[i+j+k];
			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,sz(b)) in[i].imag(b[i]);
	fft(in);
	for(C& x: in) x *= x;
	rep(i,n) out[i] = in[-i & (n-1)] - conj(in[i]);
	fft(out);
	rep(i,sz(res)) res[i] = imag(out[i]) / (4*n);
	return res;
}

#define S 65536

vd tree[2*S+7];

vector<pii> cum;

void gett(int l, int r, int ll, int rr, int w) {
	if(l > rr || r < ll)
		return;
	if(l >= ll && r <= rr) {
		cum.pb({r-l+1, w});
		return;
	}
	int sr = (l+r)/2;
	gett(l,sr,ll,rr,w*2);
	gett(sr+1,r,ll,rr,w*2+1);
}

vd mult(int l, int r) {
	cum.clear();
	gett(1,S,l,r,1);
	sort(all(cum));
	vd tmp = tree[cum[0].second];
	for(int i = 1; i < cum.size(); i++) {
		tmp = conv(tmp, tree[cum[i].second]);
	}

	return tmp;
}

int n,t;

ld calc(vd &V, int k) {
	ld p = 0;
	for(int i = 0; i < V.size(); i++) {
		int score = i - (k-i);
		if(score >= t)
			p += V[i];
	}
	return p;
}


int32_t main() {
	cin.tie(0)->sync_with_stdio(0);
	cout << fixed << setprecision(10);
	cin >> n >> t;
	// n = 50000;
	// t = 12500;
	
	vd V;

	// default_random_engine gen;
    // uniform_real_distribution<double> dist(0.0, 1.0);


	for(int i = 1; i <= n; i++) {
		double x;
		cin >> x;
		// x = dist(gen);
		V.pb(x);
	}
	sort(all(V));
	reverse(all(V));

	for(int i = 0; i < V.size(); i++) {
		tree[i+S] = {1-V[i], V[i]};
	}
	for(int i = V.size(); i <= S; i++) {
		tree[i+S] = {1};
	}
	for(int i = S-1; i >= 1; i--) {
		tree[i] = conv(tree[i*2], tree[i*2+1]);
	}
	

	int C = max(1,4*(int)(sqrt(n-t)));
	int K = 2;

	int b = t;
	int where = b;
	vd tmp = mult(1,b);
	ld maks = calc(tmp, b);

	for(int i = t+1; i <= min(n,t + K*C); i++) {
		vd tmp2;
		tmp2.resize(i+1);
		ld p = V[i-1];
		tmp2[0] = tmp[0] * ((ld)1-p);
		tmp2.back() = tmp.back() * p;
		for(int j = 1; j < i; j++) {
			tmp2[j] = tmp[j-1] * (p) + tmp[j] * ((ld)1-p);
		}
		swap(tmp, tmp2);
		// ans = max(ans, calc(tmp, i));
		ld pp = calc(tmp, i);
		if(pp > maks) {
			maks = pp;
			where = i;
		}
	}

	tmp = mult(1,b);

	while(1) {
		if(b == n)
			break;

		int lastt = b;
		b += C;
		b = min(b,n);

		vd tmp2 = mult(lastt+1, b);
		tmp = conv(tmp, tmp2);

		ld p = calc(tmp, b);
		if(p > maks){
			maks = p;
			where = b;
		}
	}

	
	int s = max(t, where-K*C);
	tmp = mult(1,s);
	ld ans = calc(tmp, s);
	for(int i = s+1; i <= min(n,where+K*C); i++) {
		vd tmp2;
		tmp2.resize(i+1);
		ld p = V[i-1];
		tmp2[0] = tmp[0] * ((ld)1-p);
		tmp2.back() = tmp.back() * p;
		for(int j = 1; j < i; j++) {
			tmp2[j] = tmp[j-1] * (p) + tmp[j] * ((ld)1-p);
		}
		swap(tmp, tmp2);
		ans = max(ans, calc(tmp, i));
	}
	cout << ans << "\n";
}

/*
5 2
0.77
0.85 
0.75 
0.98 
0.6

*/