function main(input) {
    var n = input.split(/\n/)[0] / 1;
    var a = input.split(/\n/)[1];
    var as = a.split(/\s/);
    for (var i = 0; i <= n - 1; i++) {
        as[i] /= 1;
    }
    var result = 0;
    for (var j = 0; j <= n - 1; j++) {
        result += 1 / as[j];
    }
    result = 1 / result;
    console.log(result);
}

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