
function main(arg) {
  const input = arg.split('\n');
  const ary = input[1].split(' ');
  
  let tmp = 0;
  for(let i = 0; i <input[0]; i++){
    tmp += 1/ary[i];
  }
  console.log(1/tmp);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));