let solve s =
  let n = String.length s in
  let check i =
    if s.[i] = s.[n-1-i]
    then 0
    else 1 in
  let rec iter i res =
    if i = n/2
    then res
    else iter (i+1) (res + check i)
  in iter 0 0

let () = Scanf.scanf "%s" @@ fun s -> Printf.printf "%d\n" @@ solve s 