inputs = readlines
# inputs = "3\n200 200 200\n".split("\n")

n = inputs[0].to_i
as = inputs[1].split(' ').map(&:to_i)

def solve(n, as)
  asum = as.inject(0) do |sum, a|
    sum + (1.0 / a)
  end
  1.0 / asum
end

puts solve(n, as)
