function Main(input) {
  const inputs = input.split("\n");
  const n = parseInt(inputs[0], 10);
  var nums = new Array(n)
  nums = inputs[1].split(" ");
  const result = 1 / nums.map(num => 1 / parseInt(num, 10))
    .reduce((acc, value) => acc + value);
  console.log(result);
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));