#include <bits/stdc++.h>
int gcd(int x, int y) {
  int z;
  while (y != 0) {
    if (y < x) {
      z = x;
      x = y;
      y = z;
    }
    y = y % x;
  }
  return x;
}
void add(int sum[2], int a) {
  int num = gcd(a * sum[0] + sum[1], sum[1] * a);
  sum[0] = (a * sum[0] + sum[1]) / num;
  sum[1] = (sum[1] * a) / num;
}
int main() {
  int i, N, num;
  scanf("%d", &N);
  int m[N], sum[2];
  sum[0] = 0;
  sum[1] = 1;
  for (i = 0; i < N; i++) {
    scanf("%d", &m[i]);
    add(sum, m[i]);
  }
  printf("%lf\n", (double)sum[1] / sum[0]);
  return 0;
}
