object Main extends App{
	val sc = new java.util.Scanner(System.in)
	val n = sc.nextInt

	def calc(n1: BigDecimal, n2: BigDecimal): Stream[BigDecimal] = {
		n1 #:: calc(n2, (19/4.0 * n2) - (12 / 4.0 * n1) )
	}
	val res = calc(4, 3).take(n).last
	println(res)
}