function Main(input) {
	input = input.split("\n");
	tmp = input[0].split(" ");
	var number = input[1].split(" ");
	var answer = 0;
	for(var i=0;i<tmp;i++){
      answer = answer + 1/number[i];
    }
	answer = 1/answer;
	console.log(answer);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));