'use strict'

function main(input) {
  input = input.trim().split('\n')
  const n = input[0]
  const ans = input[1]
    .split(' ')
    .map(v => parseInt(v, 10))
    .map(v => 1 / v)
    .reduce((a, b) => a + b)
  console.log(1 / ans)
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'))
