'use strict'
const main = (N,A) => {
    const result = A.reduce((x,i)  => {return x+=(1/i)},0)
    return 1/result
}
const input = require('fs').readFileSync('/dev/stdin','utf8').split("\n")
const N = Number(input[0])
const A = input[1].split(" ").map( x => Number(x))
console.log(main(N,A))