#include<bits/stdc++.h>
#define inf 1e18
#define Endl endl
using namespace std;
#define int long long
#define PII pair<int,int>
int dxx[] = {-1, 1, 0, 0}; // 0上 1下
int dyy[] = {0, 0, -1, 1}; // 2左 3右
#define all(a) a.begin()+1, a.begin()+1+n
int gcd(int a,int b){return b==0?a:gcd(b,a%b);}
int lcm(int a,int b){return a*b/gcd(a,b);}
#define ios ios::sync_with_stdio(false), cin.tie(0), cout.tie(0)
const int N=2e5+10,mod=998244353;
void solve(){
    int n;
    cin>>n;
    vector<double> a(n+10);
    for (int i=1;i<=n;i++){
        cin>>a[i];
    }
    double ans=0;
    for (int i=1;i<=n;i++){
        double now=0;
        for (int j=1;j<=n;j++){
            if(i==j)
                continue;
            now+=a[j]/(a[i]+a[j]);
        }
        now*=a[i];
        ans+=now;
    }
    cout<<ans<<endl;
}
signed main(){
    ios;
    int T=1;
    cout<<fixed<<setprecision(10);
//     cin>>T;
    while (T--){
        solve();
        if(T!=0)
           cout<<endl;
    }
    return 0;
}
//Shift+Alt+F格式化
//⠀⠀⠀⢛⢆⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⣀⠤⠀⠀⠂⣠⡀⢀⣀⠀⠠⢄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠠⡄⠀⠀⠀
//⠀⠀⠀⠀⠀⠈⠀⠀⠀⠀⠀⠀⠀⢀⢤⢤⡘⠀⠀⠀⠀⠔⠋⠘⢆⠀⠙⢦⠀⠑⠢⡀⠀⠀⠀⠀⠀⠀⠀⠀⠃⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠣⠀⠀⠀⠀⠠⢢⠃⠀⠈⠀⠀⠀⠀⠀⠀⠉⢿⣷⡀⠀⠑⡄⠀⠈⢢⠀⠀⢀⡀⠀⢰⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠀⡀⠀⠀⠰⠡⡟⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⢻⣧⠀⠀⠙⠀⠀⠀⣇⠀⠀⠀⠀⠚⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠰⣇⠀⠰⠁⠀⠀⠀⠀⣴⠀⠀⠀⠀⠰⢧⠀⠀⠘⣿⡄⠀⠀⠀⠀⠰⡌⣱⡄⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠤⠀⠀⠉⠀⡁⠀⠀⢠⠀⠀⢎⡆⠀⠀⠀⠐⡤⠵⢖⢲⣿⣇⠀⠀⠰⣦⣦⡈⢫⣃⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⢠⡀⠀⠀⢀⡅⣤⠀⣼⠒⠞⣀⡣⣤⣤⣤⣴⢷⣤⣤⠷⣮⢿⠀⠀⠀⠀⠈⠹⡰⠳⠁⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⠃⣿⣀⣿⡾⢷⡖⢶⡄⠉⠁⠈⠀⢏⠁⠉⠹⢘⠀⠀⠀⠀⠀⠀⣼⠷⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⡏⣿⡄⠘⠧⡁⠀⠇⠀⠀⠀⠀⠈⠀⠀⠀⠈⠀⠀⣾⣦⣶⣤⣿⣦⠲⡀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠀⠀⠘⣠⢁⢹⣇⠀⠀⠀⠀   ~~~     ⠈⣠⠀⠀⣿⣿⣿⣿⡿⣿⣧⡘⢄⠀⠈⠀⠀⠀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠁⣾⢸⣿⣦⣀⠂⠀⠀⠀⠀⠀⠀⠀⠀⠔⠊⢹⠀⠀⡹⠛⠟⠁⠠⣇⠛⠿⢦⡑⢄⠀⠀⠄⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠀⠈⠀⠀⢠⣈⣸⡟⠁⠈⡹⠛⠉⠉⠉⢒⡀⢔⠁⠀⠀⢸⣿⣿⣷⠀⠀⢀⡠⡚⢦⣒⣄⠈⠀⠑⠤⡀⠀⠀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⠐⠁⢀⡠⢺⣿⣿⡟⠀⠀⠈⠢⠇⠀⡀⢺⡶⣾⣕⣄⡀⣼⣿⣿⡿⠀⠉⠁⠀⠘⡄⠱⢿⣿⣄⡀⠑⠌⠢⡀⠀⠀⠀⠀
//⠀⠀⠀⠀⠀⠀⢀⠴⣋⢠⣾⣿⣿⡁⠀⠀⠀⠀⠀⡸⢁⣿⠇⣿⢻⣷⡍⢾⣿⣿⣿⡄⠀⠀⠀⠀⣸⢧⠈⢻⣿⣿⣦⣀⣀⢠⠀⠀⠀⠀