#include <bits/stdc++.h>
using namespace std;


int main(){

    int n;
    cin>> n;
    long double temp,ans=0;
    for(int i= 0; i<n; ++i){
        cin>> temp;
        ans+= 1/temp;
    }
    cout<<setprecision(20)<<endl;
    cout<< 1/ans <<endl;
}