function Main(input) {
    input = input.split("\n")
    var N = input[0];
    var A = input[1].split(" ");
    var ans = [];
    for(var i = 0; i < N; i++){
        ans.push(1 / A[i]);
    }
    var res = 1 / ans.reduce((a,x) => a+=x,0);
    console.log(res);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));