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 () {
  var count = lines[0];
  var array = lines[1].split(' ');
  var ammount = 0;
  for (var i=0;i < count;i++) {
    ammount += 1 / array[i];
      
  }
  console.log(1 / ammount);
});