using System;
using System.Collections.Generic;
using System.Linq;
using System.Security.AccessControl;
using System.Security.Cryptography;
using System.Text;
using System.Threading.Tasks;

namespace Atcoder
{
    class Program
    {
        static void Main(string[] args)
        {
            var n = ReadLineInt();
            var a = ReadLineIntSpilit();

            var e = 0.0;
            for (int i = 0; i < n; i++) {
                e += 1.0 / a[i];
            }

            Console.WriteLine(1.0 / e);
            ReadLine();
        }

        //メモ

        //アルゴリズム

        //・全探索解

        /*
         * 例 [A, B, C]
         *
         *  A - B, A - C
         *  B - C
         * 
            for (int i = 0; i < n; i++) {
           
                for (int j = i + 1; j < n; j++) {

                    array[i], array[j]

                }
            }
        */

        // ・配列の前後を見る

        /*
         *  例 array[1, 2, 3, 4, 5]
         *  
         *  r = 3;
         * 
         *  array[i - 1] => 1
         *  array[i] => 2
         *  array[i + 1] => 3
         * 
         * //範囲
         * int r = 3;
         * 
         * for(int i = 1; i <= array.Length - r; i++) { }
         * 
         */

        //配列
        
            //降順
            //var sort = array.OrderBy(x => -x).ToArray();

            //合計値
            //array.Sum();

            //最大値
            //List.Sort(); List[Count() - 1] 

        //切り捨て

            //double d = Math.Floor(n);

        //切り上げ
       
            //double d = Math.Celling(n);

        //kのn乗
        //(int) Math.Pow(k, n);

        //for文の逆
        //for(int i = array.Length - 1; i >= 0; i--) { }


        /// <summary>
        /// 最大公約数
        /// </summary>
        static int gcd(int a, int b)
        {
            if (a > b) {
                return gcd(b, a);
            }

            while (b != 0) {
                var remainder = a % b;
                a = b;
                b = remainder;
            }

            return a;
        }

        //最小公倍数
        static int lcm(int a, int b) => a * b / gcd(a, b);

        /// <summary>
        /// コンビネーション
        /// </summary>
        static int nCm(int n, int m)
        {
            if (m == 0) return 1;
            if (n == 0) return 0;
            return n * nCm(n - 1, m - 1) / m;
        }

        /// <summary>
        /// 数字配列の重複をカウント
        /// </summary>
        public static int DistinctInt(IEnumerable<int> array)
        {
            var q = array.Distinct().Count();
            return array.Count() - q;
        }

        /// <summary>
        /// 文字配列の重複をカウント
        /// </summary>
        /// <param name="array"></param>
        /// <returns></returns>
        public static int DistinctString(IEnumerable<string> array)
        {
            var q = array.Distinct().Count();
            return array.Count() - q;
        }

        public static string ReadLine() => Console.ReadLine();
        public static int ReadLineInt() => int.Parse(Console.ReadLine());
        public static long ReadLineLongInt() => long.Parse(Console.ReadLine());
        public static string[] ReadLineStringSplit(char a = ' ') => ReadLine().Split(a).ToArray();
        public static int[] ReadLineIntSpilit(char a = ' ') => ReadLine().Split(a).Select(x => int.Parse(x)).ToArray();
        public static long[] ReadLineLongSpilit(char a = ' ') => ReadLine().Split(a).Select(x => long.Parse(x)).ToArray();
        static string[] GetAlphaLowerArray() => new string[] { "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z" };
        static string[] GetAlphaUpperArray() => new string[] { "A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z" };
    }
}


Measured memory_bytes: 13396