import           Control.Monad
import qualified Data.ByteString.Char8 as BS
import           Data.List
import           Data.Maybe

readInts :: IO [Int]
readInts = map (fst . fromJust . BS.readInt) . BS.words <$> BS.getLine

main = do
  str <- BS.unpack <$> BS.getLine
  print $ solve str (reverse str) `div` 2

solve :: String -> String -> Int
solve [] [] = 0
solve (a:xa) (b:xb)
  | a /= b = 1 + solve xa xb
  | otherwise = solve xa xb
