'use strict';

function main(stdin) {
  const input = stdin.split('\n');
  const N = Number(input.shift());
  const params = input[0].split(' ').map(v => parseInt(v, 10));

  let sum = 0;
  params.forEach(p => {
    sum += 1 / p;
  });
  
  console.log(1 / sum);
}

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