main = do
  n <- readLn
  putStrLn $ show $ luca n

luca :: Int -> Int
luca 0 = 2
luca 1 = 1
luca n = (lucalist !! (n - 1)) + (lucalist !! (n - 2))

lucalist :: [Int]
lucalist = [ luca i | i <- [0,1..] ]