f' :: Char -> String -> String
f' c [] = [c]
f' c (c' : cs)
  | c == c' = c : f' c cs
  | otherwise =  [c]

f :: String -> String
f [] = undefined
f (c : cs) = c : f' c cs

main :: IO ()
main = do
  s <- getLine
  putStrLn (f s)