"use strict";
const main = arg => {
  let Nums = arg.split("\n");
  let NumArr = Nums[1].split(" ").map(x=> x*1);
  let MotuerSum = 0;
  for(let i = 0; i < Nums[0]; i++){
  	MotuerSum += 1/NumArr[i];
  }
  console.log(1/MotuerSum);
}

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