function main(input) {
  const inputArray = input.split(/\n/);
  const N = +inputArray.shift();
  const numberArray = inputArray[0].split(/\s/);
  const sum = numberArray.reduce((a,c) => a += 1/+c, 0);
  console.log(1/sum);
}
 main(require('fs').readFileSync('/dev/stdin', 'utf8'));