counta :: String -> Int
counta [] = 0
counta (c : cs)
  | c == 'a' = 1 + counta cs
  | otherwise = 0 + counta cs

f :: String -> String
f s = take (counta s + 1) ['a', 'a'..]

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