"use strict"

const arg_str = require('fs').readFileSync('/dev/stdin', 'UTF-8')
const args_lines = arg_str.split('\n')
const n = args_lines[0]

const a_array = args_lines[1].split(' ')
const a_array_invert = a_array.map(x=>1/parseInt(x))
const sum = a_array_invert.reduce((a, b)=>a + b)

console.log(1/sum)