n = gets.to_i
a = Array.new(n + 1, 0)
a[0] = Rational(4)
a[1] = Rational(3)
2.upto(n) do |i|
    a[i] = Rational(19, 4) * a[i - 1] - Rational(3) * a[i - 2]
end
puts a[n].to_f