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#include <bits/stdc++.h>
using namespace std;

#define FOR(i,l,r) for(int i = (l); i <= (r); i++)
#define FORD(i,l,r) for(int i = (l); i >= (r); i--)

using num = double;
using ind = long long;

double prob_board[50'005];
int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    ind n,t;
    cin >> n >> t;
    vector<num> v;
    v.resize(n);
    FOR(i,1,n){
        cin >> v[i-1];
    }
    v.push_back(9);
    sort(v.begin(),v.end(), [](num a, num b){
        return a > b;
    });
    prob_board[0]=1;
    num wyn_max = 0;

    ind last_req = t + (n-t)/2 + ((n-t)%2);
    num after_last_req = 0;
    FOR(i,1,n){
        num pnow = v[i];
        num p_now = 1.0 - pnow;
        ind the_end = 1;
        the_end = max(the_end, last_req - (n-i)-2);//+n/5);
        ind the_begin = i;
        the_begin = min(the_begin, last_req);
        after_last_req += prob_board[last_req]*pnow;
        FORD(j,the_begin,the_end){
            prob_board[j] = prob_board[j-1]*pnow + prob_board[j]*p_now;
        }
        prob_board[0] *= p_now;
        if(i < t) continue;
        num wyn_now = 0;
        FOR(j, t + ((i-t)/2) + ((i-t)%2),i){
            wyn_now += prob_board[j];
        }
        wyn_now += after_last_req;
        wyn_max = max(wyn_max, wyn_now);
    }
    //cout << std::setprecision(16) << std::fixed << wyn_max << '\n';
    cout << std::setprecision(16) << std::fixed << wyn_max << '\n';
}