const Main = (input) => {
  const dataArray = input.split("\n");
  const count = dataArray[0];
  const numberArray = dataArray[1];
  var sumNumber;
  for (var i = 0; i < numberArray.length; i ++){
  	var sumNumber += 1/numberArray[i];
  }
  var answer = 1/sumNumber;
  
  console.log(answer);
  
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));