open System

let inline (+=) (x: byref<'a>) (y: 'a) = x <- x + y

let solve n =
  let dp = Array.zeroCreate (n + 1)
  dp.[n] <- 1L
  for i in (n - 1) .. -1 .. 0 do
    if (i + 2) <= n then &dp.[i] += dp.[i + 2]
    if (i + 1) <= n then &dp.[i] += dp.[i + 1]
  dp.[0]

let N = Console.ReadLine() |> int

solve N
|> Console.WriteLine