process.stdin.resume();
process.stdin.setEncoding('utf8');
// Your code here!

var lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});
reader.on('line', function (line) {
  lines.push(line);
});
process.stdin.on('end', function () {
  const count = lines[0];
  const array = lines[1].split(' ');
  let ammount = 0;
  for (let value of array) {
    ammount += 1 / value;
  }
  console.log(1 / ammount)
});