'use strict'

const main = (INPUT) => {
  const input = INPUT.split('\n')
  const N = input[0]
  const A = input[1].split(' ')
  let ans = 0
  for (let i = 0; i < A.length; i++) {
    ans += 1 / A[i]
  }
  console.log(1 / ans)
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'))
