#include <map>

#include <set>

#include <list>

#include <cmath>

#include <queue>

#include <stack>

#include <cstdio>

#include <vector>

#include <string>

#include <cctype>

#include <complex>

#include <cassert>

#include <utility>

#include <cstring>

#include <cstdlib>

#include <iostream>

#include <algorithm>

using namespace std;

typedef pair<int,int> PII;

typedef long long ll;

#define lson l,m,rt<<1

#define pi acos(-1.0)

#define rson m+1,r,rt<<11

#define All 1,N,1

#define read freopen("in.txt", "r", stdin)

const ll  INFll = 0x3f3f3f3f3f3f3f3fLL;

const int INF= 0x7ffffff;

const int mod =  ;

double dp[],p[];

int n;

void solve(){

    int cas=(<<n)-;

    dp[cas]=;

    for(int i=cas-;i>=;--i)

    {

        dp[i]=;

        double tmp=0.0;

        for(int j=;j<n;++j)

        if(!(i&(<<j))){

            tmp+=p[j];

            dp[i]+=dp[i+(<<j)]*p[j];

        }

        dp[i]/=tmp;

    }

    printf("%lf\n",dp[]);

}

int main()

{

    while(~scanf("%d",&n)){

        for(int i=;i<n;++i)

            scanf("%lf",&p[i]);

        solve();

    }

return ;

}