require 'time'

def calcs(x,y,ax,ay,bx,by) ((ax-x)*(by-y)-(bx-x)*(ay-y))/2.0 end

def inpf() a=gets.chomp.split(" ").map(&:to_f)end

def inps() a=gets.chomp.split(" ")end

def copy(a) Marshal.load(Marshal.dump(a)) end

def factorial(n)(n < 2)? 1 : (2..n).inject(:*) end

def scount(a) b=na(a.max+1);a.each{|n|b[n]+=1};return b end

def na(n,d=0) Array.new(n,d)end

def na2(n,m,d=0) Array.new(n){Array.new(m,d)}end

def na3(n,m,l,d=0) Array.new(n){Array.new(m){Array.new(l,d)}}end

def yn(f) return (f)? "Yes": "No" end

def ynb(f) return (f)? "YES": "NO" end

def ap(a) a.each{|u| p u} end

def bit(n) n.to_s(2).split("").map(&:to_i) end

def inp() a=gets.chomp.split(" ").map(&:to_i)end

n = inp[0]#k,f = inp
a = inp
bo = Rational(0,1)
n.times do |i|
  bo += Rational(1,a[i])
end
p (1.0/bo)
#puts ()? "Yes" : "No"
#puts ()? "YES" : "NO"
#puts ()? "yes" : "no"