1 #include<iostream>
 2 #include<cstring>
 3 #include<queue>
 4 #include<cstdio>
 5 #include<stack>
 6 #include<set>
 7 #include<map>
 8 #include<cmath>
 9 #include<ctime>
10 #include<time.h> 
11 #include<algorithm>
12 using namespace std;
13 #define mp make_pair
14 #define pb push_back
15 #define debug puts("debug")
16 #define LL long long 
17 #define pii pair<int,int>
18 #define eps 1e-10
19 #define inf 0x3f3f3f3f
20 
21 double f[(1<<21)+30];
22 double p[25];
23 int main()
24 {
25     int t,i,j,k,n,m,u,v;
26     while(scanf("%d",&n)==1){double none=0;
27         for(i=0;i<n;++i) scanf("%lf",p+i),none+=p[i];
28         int all=(1<<n)-1;
29         memset(f,0,sizeof(f));
30         for(i=all-1;i>=0;--i){
31             double Pj=(double)1.00-none,s=0;
32             for(j=0;j<n;++j){
33                 if(i&(1<<j)){
34                     Pj+=p[j];
35                 }
36                 else{
37                     s+=p[j]*f[i|(1<<j)];
38                 }
39             }
40             s+=1.00;
41             Pj=1.00-Pj;
42             f[i]=s/Pj;
43         }
44         printf("%.5f\n",f[0]);
45     }
46     return 0; 
47 }