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#include <bits/stdc++.h>
#define fi first
#define sc second
#define pb push_back
#define rep(i,p,k) for(int i=(p);i<(k);++i)
#define forn(i,p,k) for(int i=(p);i<=(k);++i)
#define forr(i,k,p) for(int i=(k)-1;i>=(p);--i)
#define each(a,b) for(auto&a:b)
#define all(v)  begin(v), end(v)
#define RET(smth)   return void(cout<<(smth)<<'\n');
#define sz(v) (int)v.size()
using namespace std;
using pii = pair<int,int>;
using ll = long long;
using lll = __int128_t;
using Vi = vector<int>;
using ld = double;
#ifdef DEBUG
#include "debug.h"
#else
#define debug(...) ;
#endif

const ld eps = 1e-9;

int main() {
#ifndef DEBUG
    cin.tie(0) -> sync_with_stdio(0);
#endif
    int n,t; cin >> n >> t;
    vector<ld> pr(n);
    for(auto &p : pr) cin >> p;
    sort(all(pr));
    while(!pr.empty() && pr.back() == 1) {
        pr.pop_back();
        t--;
    }
    reverse(all(pr));
    while(!pr.empty() && pr.back() == 0) pr.pop_back();
    n = sz(pr);
    if(t <= 0) {
        cout << 1 << '\n';
        return 0;
    }
    if(t > n) {
        cout << 0 << '\n';
        return 0;
    }
    vector<ld> poly(n+1);
    auto mul = [&](ld p1, ld p2, int deg) {
        auto a = p1*p2;
        auto b = p1 + p2 - 2*a;
        auto c = 1 - p1 - p2 + a;
        for(int i=deg;i>=0;i--) poly[i+2] = poly[i]*a + poly[i+1]*b + poly[i+2]*c;
        poly[1] = poly[0]*b + poly[1]*c;
        poly[0] *= c;
    };
    ld res = 0;
    if(t == 1) res = pr[0];
    if(t % 2) {
        poly[0] = 1 - pr[0];
        poly[1] = pr[0];
    } else {
        poly[0] = 1;
    }
    for(int i=t%2;i+1<n;i+=2) {
        mul(pr[i], pr[i+1], i);
        if(i + 2 >= t) {
            ld pp = 0;
            forn(j,(t+i+2)/2,i+2) pp += poly[j];
            if(pp + 1e-7 < res) break;
            res = max(res,pp);
            // res = pp;
        }
    }
    cout << setprecision(9) << fixed << res << '\n';
    return 0;
}