function Main(input) {
  input = input.trim()
  input = input.split("\n");
  var N =input[0];
  var arr = input[1].split(" ").map(Number);
  var sum=0
  for(var i=0;i<N;i++){
    sum += 1/arr[i];
  }
  console.log(1/sum);
}Main(require("fs").readFileSync("/dev/stdin", "utf8"));