import std.stdio, std.string, std.array, std.conv;

void main() {
    int n = readln.chomp.to!int;
    int[] a = readln.split.to!(int[]);
    double sum = 0;
    foreach (x; a) {
        sum += 1.0 / x;
    }
    writefln("%.8f", 1.0 / sum);
}