n = gets.to_i
a = gets.split.map(&:to_i)
inv = 0
b = 100000000
a.each do |num|
    inv += b/num
end
puts (b*b/inv)/b.to_f

=begin
n,m = gets.split.map(&:to_i)
puts ">"*(m*7+3)+"<<<<<<<[#{">"*(n*7)}+#{"<"*(n*7)}-]"*m+">"*(n*7-3)+"+"*n+"[-[<<<<<<<+>>>>>>>-]<<<<"+">>>>>>>[<<<<<<<+>>>>>>>-]"*m+"<"*(m*7+2)+"+[>+>"+"[>+>>+<<<-]>>>[<<<+>>>-]<<[->[>>+<]>[<]>-[>>>>+<<<<<<++++++++++>>+]<<-<]>>>>>>"*(m+1)+">["+"<"*(m*7+10)+"->->"+"[>+>>+<<<-]>>>[<<<+>>>-]<<[->---------[>>+<]>[<]>-[>>>>+<<<<<<---------->>+]<<++++++++++<]>>>>>>"*(m+1)+">[-]]"+"<"*(m*7+10)+"]<]"


# a/b
# n = ceil(log_10 a)
# m = ceil(log_10 b)
# 
# a = a_0+a_1*10+......a_(n-1)*10**(n-1)
# b = b_0+b_1*10+......b_(n-1)*10**(m-1)
#
# 7 cells = 1 digit
# [*][][][b_0][][a_0][] [][][][b_1][][a_1][] ......
# 
# [*][][quo_0][][][mod_0][] [][][quo_1][][][mod_1][] ......

=end