import System.Environment
import Data.List
import Text.Printf

fib :: Int -> Int
fib 0 = 0
fib 1 = 1
fib n = fib (n-1) + fib (n-2)

main :: IO ()
main = do
  args <- getArgs
  case args of
    [aInteger] | [(n,_)] <- reads aInteger ->
      printf "%d\n" (fib n)