#include <bits/stdc++.h>
int main(void) {
  int i, n, a[110];
  double A, b;
  scanf("%d", &n);
  for (i = 0; i < n; i++) scanf("%d", &a[i]);
  for (i = 0; i < n; i++) A += (double)1 / a[i];
  b = 1 / A;
  printf("%.5lf\n", b);
  return 0;
}
