#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  double a[101];
  double x;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  for (int i = 0; i < n; ++i) {
    x += 1 / a[i];
  }
  cout << 1 / x << endl;
  return 0;
}
