N = gets.to_i

A = Array.new(N + 1, 0)
A[0] = 4
A[1] = 3

2.upto(N) do |k|
  A[k] = Rational(19 * A[k - 1] - 12 * A[k - 2], 4)
end

puts "%.12f" % A[N]