main :: IO ()                                                                                                              
main = readLn >>= print . lucas 2 1                                                                                        
                                                                                                                           
lucas :: (Integral a) => a -> a -> a -> a                                                                                  
lucas a _ 0 = a                                                                                                            
lucas _ b 1 = b                                                                                                            
lucas a b n = lucas b (a+b) (n-1)