'use strict';
const main = input => {
  const n = parseInt(input[0], 10);
  let a = input[1].split(' ').map(val => parseInt(val, 10));
  // console.log(a);
  a = a.map(val => 1/val);
  // console.log(a);
  let sum = a.reduce((sum, data) => sum + data);
  // console.log(sum);
  let ans = 1/sum;
  console.log(ans);
  
};
main(require("fs").readFileSync("/dev/stdin", "utf8").split('\n'));