#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for(int i = s; i < t; ++i)
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define sz(x) (int)(x).size()
#define v(T) vector<T>
#define vv(T) v(v(T))
#define in(e,l) (std::find(l.begin(), l.end(), e) != l.end())
#define asort(l) (std::sort(l.begin(), l.end()))
#define dsort(l) (std::sort(l.rbegin(), l.rend()))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;

int main() {
  ll N;
  cin >> N;
  vl a(N);
  double ans = 0;
  rep(i, N){
    cin >> a[i];
    ans += (1.0) / a[i];
  }
  cout << 1 / ans << endl;
  return 0;
}

