/* 4336 */

 #include <iostream>

 #include <sstream>

 #include <string>

 #include <map>

 #include <queue>

 #include <set>

 #include <stack>

 #include <vector>

 #include <deque>

 #include <algorithm>

 #include <cstdio>

 #include <cmath>

 #include <ctime>

 #include <cstring>

 #include <climits>

 #include <cctype>

 #include <cassert>

 #include <functional>

 #include <iterator>

 #include <iomanip>

 using namespace std;

 //#pragma comment(linker,"/STACK:102400000,1024000")

 #define sti                set<int>

 #define stpii            set<pair<int, int> >

 #define mpii            map<int,int>

 #define vi                vector<int>

 #define pii                pair<int,int>

 #define vpii            vector<pair<int,int> >

 #define rep(i, a, n)     for (int i=a;i<n;++i)

 #define per(i, a, n)     for (int i=n-1;i>=a;--i)

 #define clr                clear

 #define pb                 push_back

 #define mp                 make_pair

 #define fir                first

 #define sec                second

 #define all(x)             (x).begin(),(x).end()

 #define SZ(x)             ((int)(x).size())

 #define lson            l, mid, rt<<1

 #define rson            mid+1, r, rt<<1|1

 const int maxn = ;

 double p[maxn];

 double dp[<<maxn];

 int n;

 void solve() {

     int mst =  << n;

     int mask = mst - ;

     dp[mask] = ;

     per(i, , mask) {

         double tmp = 0.0;

         dp[i] = 1.0;

         rep(j, , n) {

             if (i & (<<j))

                 continue;

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

             tmp += p[j];

         }

         dp[i] /= tmp;

     }

     double ans = dp[];

     printf("%.06lf\n", ans);

 }

 int main() {

     ios::sync_with_stdio(false);

     #ifndef ONLINE_JUDGE

         freopen("data.in", "r", stdin);

         freopen("data.out", "w", stdout);

     #endif

     while (scanf("%d", &n)!=EOF) {

         rep(i, , n)

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

         solve();

     }

     #ifndef ONLINE_JUDGE

         printf("time = %d.\n", (int)clock());

     #endif

     return ;

 }