main :: IO ()
main = do
  s <- getLine
  print $ solve s

solve :: String -> Int
solve s = sum $ zipWith f xs ys
  where xs = take n s
        ys = take n $ reverse s
        n = length s `div` 2

f :: Char -> Char -> Int
f x y
   | x /= y    = 1
   | otherwise = 0
