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#include <bits/stdc++.h>

using namespace std;
#define pb push_back
#define st first
#define nd second
typedef long long ll;
typedef long double ld;
const ll I = 1000'000'000'000'000'000LL;
const int II = 2'000'000'000;
const ll M = 1000'000'007LL;
const ld E = 0.00001;
const int N = 50'007;
const int K = 1'500;
ld tab[N];
vector<ld> ws[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 long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef complex<double> C;
typedef vector<long 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;
}

void Solve()
{
    int n, k;
    double a;
    cin >> n >> k;
    for(int i = 1; i <= n; ++i)
    {
        cin >> a; tab[i] = a;
    }
    ld answer = 0;
    sort(tab + 1, tab + 1 + n);
    reverse(tab + 1, tab + 1 + n);
    
    vector<ld> cur = {1}; int d = 0;
    cout << fixed << setprecision(3);
    for(int i = 1; i <= n; i += K)
    {
        /*cout << "S: " << i << "\n";
        for(int i = 0; i < (int)cur.size(); ++i)
            cout << cur[i] << " ";
        cout << "\n";*/

        vector<ld> akt((2 * K + 1), 0);
        int da = K;
        vector<ld> nw((2 * K + 1), 0);
        akt[da] = (ld)1;

        ld cans = 0;
        for(int j = k + K; j < i && j + d < (int)cur.size(); ++j)
            cans += cur[j + d];
        vector<ld> ca; int dd = -(k - K);
        for(int j = k - K; j <= k + K - 1; ++j)
            if(j + d < 0 || j + d >= (int)cur.size())
                ca.pb((ld)0);
            else
                ca.pb(cur[j + d]);
        vector<ld> hl((int)ca.size(), 0);
        ca.pb(0);
        for(int r = 1; r <= min(K, n - i + 1); ++r)
        {
            int j = i + r - 1;
            ld dod = ca[k + K - 1 + dd] * tab[j];
            for(int l = k - K + 1; l <= k + K - 1; ++l)
            {
                int a = l + dd;
                hl[a] = ca[a - 1] * tab[j] + ca[a + 1] * ((ld)1 - tab[j]);
                if(l >= k) dod += hl[a];
            }
            answer = max(answer, cans + dod);
            for(int l = k - K; l <= k + K - 1; ++l)
                ca[l + dd] = hl[l + dd];
        }

        for(int r = 1; r <= min(K, n - i + 1); ++r)
        {
            int j = i + r - 1;
            for(int l = -r; l <= r; l += 2)
            {
                int a = l + da;
                if(a > 0)
                    nw[a] += akt[a - 1] * tab[j];
                if(a < 2 * K)
                    nw[a] += akt[a + 1] * ((ld)1 - tab[j]);
            }
            for(int l = -r; l <= r; ++l)
                {akt[l + da] = nw[l + da]; nw[l + da] = 0;}
        }

        /*cout << "E: \n";
        for(int i = 0; i < (int)akt.size(); ++i)
            cout << akt[i] << " ";
        cout << "\n\n";*/

        if(i + K <= n)
        {
            //int siz = (int)akt.size() - 1 + (int)cur.size();
            d += da;
            nw = conv(cur, akt);
            cur = nw;
        }
    }
    cout << fixed << setprecision(9) << answer << "\n";
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    //int t; cin >> t;
    //while(t--)
        Solve();

    return 0;
}