import qualified Data.ByteString.Char8 as BS

main = do
    ss <- BS.getLine
    let m = BS.length ss `div` 2
    let (a,b) = BS.splitAt m ss
    let c = if even $ BS.length ss then b else BS.tail b
    print $ sum $ BS.zipWith eq2n a $ BS.reverse c

eq2n a b = 1 - fromEnum (a == b)