1 #include <cstdio>
 2 #include <cstring>
 3 #include <algorithm>
 4 #include <cmath>
 5 using namespace std;
 6 
 7 double p[22];
 8 double dp[22];
 9 
10 int main()
11 {
12     int n;
13     while(~scanf("%d", &n)) {
14         for(int i = 0; i < n; i++)
15             scanf("%lf", &p[i]);
16         double ans = 0;
17         double sum;
18         int cnt;
19         for(int i = 1; i < (1 << n); i++) {
20             sum = 0; cnt = 0;
21             for(int j = 0; j < n; j++) {
22                 if(i & (1 << j)) {
23                     cnt++;
24                     sum += p[j];
25                 }
26             }
27             if(cnt & 1) ans += 1.0 / sum;
28             else ans -= 1.0 / sum;
29         }
30         printf("%f\n", ans);
31     }
32     return 0;
33 }