#include <bits/stdc++.h>
using namespace std;
int n;
double a[100];
void solve() {
  double res;
  for (int i = 0; i < n; i++) {
    res += 1 / a[i];
  }
  res = 1 / res;
  cout << setprecision(16) << res << endl;
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  solve();
}
