using System;
using System.Collections.Generic;
// using System.Globalization;
using System.IO;
using System.Linq;
using System.Text;

class Program
{
    static void Main()
    {
        FastScanner sc = new FastScanner(Console.OpenStandardInput());
        int N = sc.nextInt();
        decimal[] A = sc.decimalArray(N);
        Console.WriteLine(1 / A.Sum(v => 1M / v));
        return;
    }
}

// FastScanner start
public class FastScanner
{
    public readonly Stream str;
    private readonly byte[] buf = new byte[1024];
    private int len, ptr;
    public bool isEof = false;
    public FastScanner(Stream stream)
    {
        str = stream;
    }
    private T[] enumerate<T>(int n, Func<T> f)
    {
        var a = new T[n];
        for (int i = 0; i < n; ++i) a[i] = f();
        return a;
    }
    public char nextChar()
    {
        byte b = 0;
        b = read();
        while (!isEof && (b < 33 || 126 < b)) b = read();
        return (char)b;
    }
    public string nextString()
    {
        byte b = 0;
        StringBuilder builder = new StringBuilder();
        b = read();
        while (!isEof && (b < 33 || 126 < b)) b = read();
        while (!isEof && 33 <= b && b <= 126)
        {
            builder.Append((char)b);
            b = read();
        }
        return builder.ToString();
    }
    public int nextInt()
    {
        return (isEof) ? int.MinValue : (int)nextLong();
    }
    public long nextLong()
    {
        if (isEof) return long.MinValue;
        long ret = 0;
        byte b = 0;
        var ng = false;
        do b = read();
        while (b != '-' && (b < '0' || '9' < b));
        if (b == '-')
        {
            ng = true;
            b = read();
        }
        for (; true; b = read())
        {
            if (b < '0' || '9' < b)
            {
                return ng ? -ret : ret;
            }
            else ret = ret * 10 + b - '0';
        }
    }
    public decimal nextDecimal()
    {
        if (isEof) return decimal.MinValue;
        decimal ret = 0M;
        decimal div = 1M;
        byte b = 0;
        b = read();
        while (!isEof && (b < 48 || 57 < b) && b != 45) b = read();
        bool neg = b == 45;
        if (neg) b = read();
        while (!isEof && 48 <= b && b <= 57 && b != 46)
        {
            ret = ret * 10 + b - 48;
            b = read();
        }
        if (b == 46) b = read();
        while (!isEof && 48 <= b && b <= 57)
        {
            ret += (b - 48) / (div *= 10);
            b = read();
        }
        if (neg) return -ret; return ret;
    }
    public char[] charArray(int n)
    {
        return enumerate(n, nextChar);
    }
    public string[] stringArray(int n)
    {
        return enumerate(n, nextString);
    }
    public int[] intArray(int n)
    {
        return enumerate(n, nextInt);
    }
    public long[] longArray(int n)
    {
        return enumerate(n, nextLong);
    }
    public decimal[] decimalArray(int n)
    {
        return enumerate(n, nextDecimal);
    }
    private byte read()
    {
        if (isEof)
        {
            return 0;
        }
        if (ptr >= len)
        {
            ptr = 0;
            if ((len = str.Read(buf, 0, 1024)) <= 0)
            {
                isEof = true;
                return 0;
            }
        }
        return buf[ptr++];
    }
}
// FastScanner end

Measured memory_bytes: 27656