fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


fun findAns 2 = 2
  | findAns n = 
    let
        fun findAnsAux 0 a _ = a
          | findAnsAux x a b = 
            findAnsAux (x - 1) (a + b) a
    in
        findAnsAux (n - 1) (Int.toLarge 1) (Int.toLarge 1)
    end


val () =
    let
        val n = readInt ()

        val ans = findAns n
    in
        print (LargeInt.toString ans ^ "\n")
    end