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#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3")
#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 long long ll;
typedef pair<int, int> pii;
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;
}


int n, t;
vector<double> p;




vector<vd> pol;
int il;

void precomp(int n){
    int ile = 1<<(32 - __builtin_clz(n));
    il=ile;
    pol.resize(2*ile);
    for(int i=0; i<n; i++){
        pol[ile+i] = {p[i], 1-p[i]};
    }
    for(int i=n; i<ile; i++){
        pol[ile+i] = {1, 0};
    }
    for(int i=ile-1; i>0; --i){
        pol[i] = conv(pol[2*i], pol[2*i+1]);
    }
}

double wynik(int k){
    vd res = {1};
    for(int l = il, r = il+k; l<r; l/=2, r/=2){
        if(l%2) res = conv(res, pol[l++]);  
        if(r%2) res = conv(res, pol[--r]);  
    }
    int pudla = (k-t)/2;
    double wyn=0;
    for(int i=0; i<=pudla; i++){
        wyn += res[i];
    }
    return wyn;
}

pair<bool, double> prob(int k){ //1 jesli rosnie, 0 jesli maleje
    if(k<t) return {1, 0};
    if(k>n) return {0, 0};
    double w1 = wynik(k);
    if(k+1 >= n) return {0, w1};
    double w2 = wynik(k+2);
    if(w2 > w1){
        return {1, w1};
    }
    else return {0, w1};
}

double eps = 1e-6;
int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> n >> t;
    p.resize(n);
    for(int i=0; i<n; i++){
        cin >> p[i];
    }
    
    sort(p.begin(), p.end());
    reverse(p.begin(), p.end());
    precomp(n);
    int l = t, r = n;
    if(r%2!=t%2) r--;
    while(r-l > 2){
        int m = (r+l)/2;
        if(m%2 != t%2) m--;
        int m2 = m+2;
        auto p = prob(m);
        if(p.first){
            l = m2;
        }
        else{
            r = m;
        }
    }
    double best = -1;
    for(int i = l-10; i<=r+10; i+=2){
        best = max(best, prob(i).second);
    }
    cout << fixed << setprecision(7)<<best<<"\n";
}