"use strict"

const main = (input) => {
    const _input = input.split("\n");
    const A = _input[1].split(" ").map(e => parseInt(e));
    let reciprocalNumberSum = 0;

    A.forEach((value) => {
        reciprocalNumberSum += (1/value);
    });

    console.log(1/reciprocalNumberSum);
}

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