#include <bits/stdc++.h>
using namespace std;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N;
  cin >> N;
  vector<string> vec(N);
  for (int i = 0; i < 3; ++i) cin >> vec[i];
  double res = 0;
  for (auto&& v : vec) res += 1.0 / stol(v);
  cout << (1.0 / res);
}
