N = gets.chomp.to_i
A = gets.chomp.split(" ").map(&:to_i)
sum = 0

for i in 0..N-1 do
    A[i] = Rational(1, A[i])
end

for i in 0..N-1 do 
    sum = sum + A[i]
end

puts sum.denominator.to_f/sum.numerator.to_f