#include <bits/stdc++.h>
using namespace std;
#define int long long
#define maxn 105
#define lowbit(x) (x&-x)
#define log(x) (31^__builtin_clz(x))
#define endl "\n"
double a[maxn];

void solve(){
    int n;cin>>n;
    for(int i=1;i<=n;i++){
        cin>>a[i];
    }
    double ans=0;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            if(i==j)continue;
            ans+=a[i]*a[j]/(a[i]+a[j]);
        }
    }
    cout<<fixed<<setprecision(12)<<ans<<'\n';
}

signed main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    int T=1;//cin>>T;
    while(T--){
        solve();
    }
    return 0;
}