1 #include <iostream>
 2 #include <cstdio>
 3 #include <algorithm>
 4 #include <cstring>
 5 #include <string>
 6 using namespace std;
 7 typedef long long ll;
 8 typedef unsigned long long ull;
 9 typedef long double ld;
10 #define MAX 1005
11 
12 int n;
13 int c[MAX];
14 double ans;
15 void solve()
16 {
17     for(int i=1;i<=n;i++)
18         cin >> c[i];
19     if(n==1)
20     {
21         ans = c[1];
22         return;
23     }
24     if(n==2)
25     {
26         ans = (c[1]+c[2])/2.0;
27         return;
28     }
29     ans = 0;
30     for(int i=2;i<n;i++)
31         ans += (c[i])/3.0;
32     ans+=c[1]/2.0;
33     ans+=c[n]/2.0;
34 }
35 int main()
36 {
37     while(~scanf("%d",&n))
38     {
39         solve();
40         printf("%.6f\n",ans);
41     }
42     return 0;
43 }