const Main = (input) => {
  const dataArray = input.split("\n");
  const count = dataArray[0];
  const numberArray = dataArray[1].split(' ');
  var sumNumber = 0;
  for (var i = 0; i < numberArray.length; i ++){
  	sumNumber += (1 / numberArray[i]);
  }
  var answer = 1/sumNumber;
  
  console.log(answer);
  
}
 
Main(require("fs").readFileSync("/dev/stdin", "utf8"));