n, a_array = readlines.map(&:chomp)
n = n.to_i
a_array = a_array.split(' ').map(&:to_i)

result = Rational(1, a_array[0])

(1..(n-1)).each do |i|
  result += Rational(1, a_array[i])
end
puts Rational(1, result).to_f