function Main(input) {
	var data = input.split("\n");
	var n =  data[1].split(" ").map((n) => parseInt(n, 10));
	var num = 0;
	for(var i=0;i<parseInt(data[0]);i++){
		num += 1/n[i];
	}
	console.log(1/num)
}
// Don't edit this line!
Main(require("fs").readFileSync("/dev/stdin", "utf8"));