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#pragma GCC optimize("Ofast,fast-math")
#include <bits/stdc++.h>
#define all(x) (x).begin(),(x).end()
using namespace std;

using ll = long long;
using ld = long double;

//#define int ll
#define sz(x) ((int)(x).size())

using pii = pair<int,int>;
using tii = tuple<int,int,int>;

namespace FFT {
   #define rep(a,b,c) for(int a=b;a<c;a++)
   typedef complex<double> C;
   typedef vector<ld> vd;
   typedef vector<int> vi;
   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)  /// -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 multiply(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;
   }
}

vector<ld> dividefft(vector<ld> v) {
   if(sz(v) == 0) return vector<ld>{1};
   else if(sz(v) == 1) return vector<ld>{1 - v[0], v[0]};
   
   vector<ld> L, R;
   for(int i = 0; i < sz(v); i++) 
      (i % 2? L : R).emplace_back(v[i]);
   auto A = dividefft(L);
   auto B = dividefft(R);
   return FFT::multiply(A, B);
}
vector<ld> advance(vector<ld> x, ld p) {
   vector<ld> nv(sz(x) + 1);
   for(int i = 0; i < sz(x); i++) {
      nv[i + 1] += x[i] * p;
      nv[i] += x[i] * (1 - p);
   }
   return nv;
}

ld getrez(int req, int idx, vector<ld> R) {
   ld sum = 0;
   for(int i = (idx + 1 + req) / 2; i < sz(R); i++)
      sum += R[i];
   return sum;
}

ld getrez(int req, vector<ld> tofind) {
   if(sz(tofind) < req) return -1;
   auto R = dividefft(tofind);
   return getrez(req, sz(tofind), R);
}


signed main() {
   cin.tie(0) -> sync_with_stdio(0);
   int n, req;
   cin >> n >> req;
   
   vector<ld> v(n);
   for(auto &x : v) cin >> x;
   sort(all(v), greater<ld>());
   
   if(n <= 17000) {
      vector<ld> t(1, 1);
      ld mx = 0;
      for(auto x : v) {
         t = advance(t, x);
         if(sz(t) >= req + 1) mx = max(mx, getrez(req, sz(t) - 1, t));
         //if(sz(t) % 1000 == 0) {
            //cerr << mx << '\n';
         //}
      }
      cout << setprecision(7) << fixed << mx << '\n';
      return 0;
   }
   
   vector<ld> incluse;
   int P;
   for(P = 0; P < req; P++) incluse.emplace_back(v[P]);
   for(; P + 1 < sz(v) && v[P + 1] > 0.51; P += 2) {
      incluse.emplace_back(v[P]);
      incluse.emplace_back(v[P + 1]);
      
   }
   vector<ld> extras;
   for(int i = P; i < n; i++) extras.emplace_back(v[i]);
   while(sz(extras) > 8 && rbegin(extras)[4] < 0.49) extras.pop_back();
   
   //cerr << sz(incluse) << ' ' << sz(extras) << ' ' << sz(incluse) + sz(extras) << ' ' << n << '\n';
   
   if(sz(extras) % 2 != 0) extras.pop_back();
   
   
   
   int threshold = max(1000, sz(extras) / 15);
   int CNT = 0;
   while(CNT < 2 && threshold > min(100, sz(extras) / 101)) { CNT++;
      int l = sz(extras), r = 0;
      vector<pair<ld, int>> bests;

      auto simpleeval = [&](int x) {
         auto A = incluse; copy(begin(extras), begin(extras) + x, back_inserter(A));
         auto a = getrez(req, A);
         a = (floor(a * 1e10)) * 1e-10;
         return a;
      };
   
      //for(int i = 0; i < sz(extras); i += threshold) {
         //bests.emplace_back(simpleeval(i), i);
      //}
      //sort(all(bests), greater<pair<ld, int>>());
      
      //int CL = 0, CR = 0;
      //for(auto [v, i] : bests) {
         //if(v < 1e-9) break;
         //if(i > r && CR < 4)
            //r = max(r, i + threshold - 1), CR++;
         //if(i < l && CL < 4)
            //l = min(l, i), CL++;
      //}
      l = min(l, 0);
      r = max(r, sz(extras));
      if(l > r) { threshold /= 2; continue; }
    
      //cerr << l << ' ' << r << '\n';
    
      int L = l;
      vector<ld> prereq = incluse; copy(begin(extras), begin(extras) + L, back_inserter(prereq));
      auto V = dividefft(prereq);
      
      auto eval = [&](int x) {
         auto A = vector<ld>{}; copy(begin(extras) + L, begin(extras) + x, back_inserter(A));
         A = FFT::multiply(dividefft(A), V);
         auto B = incluse; copy(begin(extras), begin(extras) + x, back_inserter(B));
         B = dividefft(B);
         
         auto a = getrez(req, x + sz(incluse), A);
         a = (floor(a * 1e10)) * 1e-10;
         return a;
      };
      
      while(r - l > 2000) {
         int m1 = l + (r - l + 1) / 3, m2 = r - (r - l + 1) / 3;
         if(m1 % 2 != 0) m1--;
         if(m2 % 2 != 0) m2++;
         
         //cerr << l << ' ' << eval(m1) << ' ' << eval(m2) << ' ' << r << '\n';
         
         if(eval(m1) > eval(m2)) r = m2;
         else l = m1;
      }
      
      ld mx = max({simpleeval(l), simpleeval(r), simpleeval(0), simpleeval(sz(extras))});
      auto partial = incluse; copy(begin(extras), begin(extras) + l, back_inserter(partial));
      auto slideV = dividefft(partial);
      
      for(int i = l; i + 1 <= r; i += 2) {
         slideV = advance(slideV, extras[i]);
         slideV = advance(slideV, extras[i + 1]);
         
         //assert(abs(getrez(req, sz(slideV) - 1, slideV) - eval(i + 2)) < 1e-0);
         
         mx = max(mx, getrez(req, sz(slideV) - 1, slideV));
         //cerr << i + 2 << ' ' << simpleeval(i + 2) << '\n';
      }
      cout<< setprecision(7) << fixed << mx << '\n';
      return 0;
         
   }
   cout << "0\n";
   return 0;
}

/**
      Binecuvanteaza Doamne Ukraina.
      * teste naspa: 1026
--
*/