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#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
using namespace std;

// Copypaste z kactl.pdf
#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<long double> C;
typedef vector<long double> vd;

long double eval(int x, int k);

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);
    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];
            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;
}

long double eps = 1e-15;

int ternSearch(int a, int b, int k) {
    assert(a <= b);
    while (b - a >= 5) {
    int mid = (a + b) / 2;
    if (-eps < eval(mid+1, k)-eval(mid, k)) a = mid; // (A)
    else b = mid+1;
    }
    rep(i,a-1,b+1) if (eval(a, k) < eval(i, k)) a = i; // (B)
    return a;
}
// Koniec copypaste

vector<vd> pol[20];
int vis[50007];
long double evals[50007];
vd zd;

long double eval(int x, int k){
    int y=x;
    if(x<=0) return 0;
    //cout << "eval " << x << ": ";
    if(vis[x]){
        //cout << evals[x] << '\n';
        return evals[x];
    }
    vd res;
    int cnt=0;
    while(y>0){
        if(y%2){
            if(res.size()==0){
                res=pol[cnt][y-1];
            }
            else{
                res=conv(res, pol[cnt][y-1]);
            }
        }
        y/=2;
        cnt++;
    }
    long double ans=0;
    for(int i=res.size()-1; i>=1; i--){
        if(2*i-res.size()+1>=k){
            ans+=res[i];
        }
        else{
            break;
        }
    }
    vis[x]=1;
    evals[x]=ans;
    //cout << evals[x] << '\n';
    return ans;
}

bool cmp(long double a, long double b){
    return a-b>0;
}

int main()
{
    int n, m, i, j, k;
    long double d;
    cin >> n >> k;
    for(i=0; i<n; i++){
        cin >> d;
        zd.push_back(d);
    }
    sort(zd.begin(), zd.end(), cmp);
    if(k%2){
        pol[0].push_back({1-zd[0], zd[0]});
    }
    for(i=k%2; i+1<n; i+=2){
        pol[0].push_back(conv({1-zd[i], zd[i]}, {1-zd[i+1], zd[i+1]}));
    }
    for(j=1; j<20; j++){
        for(i=0; i+1<pol[j-1].size(); i+=2){
            pol[j].push_back(conv(pol[j-1][i], pol[j-1][i+1]));
        }
    }
    m=ternSearch((k+1)/2, pol[0].size(), k);
    cout << fixed << setprecision(7) << max(eval(m, k), eval((k+1)/2, k)) << '\n';
    return 0;
}