#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, a[100];
  cin >> n;
  float f = 0;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    f = f + ((float)1 / a[i]);
  }
  cout << 1 / f;
  return 0;
}
