function main(input){
  input = input.split('\n');
  var N = Number(input[0]);
  var A = input[1].split(' ').map(parseFloat);
  var totalA = 0;
  A.map(v => totalA += 1/v); 

  console.log(1/totalA);
}
main(require('fs').readFileSync('/dev/stdin','utf8'));