main :: IO ()                                                                                                              
main = getLine >>= putStrLn . f                                                                                            
                                                                                                                           
f :: [a] -> [a]                                                                                                            
f [] = []                                                                                                                  
f [x] = [x]                                                                                                                
f (x:_:xs) = x:f xs