"use strict";
main(require("fs").readFileSync("/dev/stdin", "UTF-8"));

function main(input) {
  input = input.split("\n");
  const N = parseInt(input.shift());
  input = input[0].split(" ");

  const denominator = commonMult(input, N);

  let culc = 0;
  for (const n of input) {
    culc += denominator / parseInt(n);
  }

  console.log(1 / (culc / denominator));
}

function commonMult(array, length) {
  let result = 1;
  for (const n of array) {
    result *= parseInt(n);
  }
  return result;
}
