#include <bits/stdc++.h>
int main(void) {
  int n;
  scanf("%d", &n);
  double a[101];
  double sum = 0;
  double ans;
  for (int i = 0; i < n; i++) {
    scanf("%lf", &a[i]);
    a[i] = 1 / a[i];
    sum += a[i];
  }
  ans = 1 / sum;
  printf("%f", ans);
  return 0;
}
