import std.stdio, std.math, std.algorithm, std.array, std.string, std.conv, std.container, std.range;
pragma(inline, true) T[] Reads(T)() { return readln.split.to!(T[]); }
alias reads = Reads!int;
pragma(inline, true) void scan(Args...)(ref Args args) {
    string[] ss = readln.split;
    foreach (i, ref arg ; args) arg = ss[i].parse!int;
}

void main() {
    readln;
    int[] a = reads();
    double b = 0;
    foreach (i; a)
        b += 1.0 / i;
    writefln("%.10f", 1/b);
}
