#include <bits/stdc++.h>
using namespace std;
long long arr[100005];
map<long long, vector<long long>> m1;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  double p = 1.0;
  long long i, j;
  long long n;
  cin >> n;
  double a[n];
  for (i = 0; i < n; i++) {
    cin >> a[i];
    p = p * a[i];
  }
  double sum1 = 0;
  for (i = 0; i < n; i++) {
    sum1 += (p / a[i]);
  }
  cout << p / sum1 << endl;
  return 0;
}
