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