object Main extends App {
  val sc = new java.util.Scanner(System.in)
  var n = sc.nextInt()
  val a:BigInt = 2
  var list = List(a,a-1)
  for (i <- 2 to 86){
    list :+= list(i-2) + list(i-1)
  }
  println(list(n))
}
