(* Examples *)
(*
stdin.ReadLine()
printfn "%d"
printfn "%s"
*)

(* Library *)
module Library = ()

(* Standard Input *)
let readSplit elementFun=
  stdin.ReadLine().Split()
  |> Array.map elementFun

let read elementFun =
  stdin.ReadLine() |> elementFun

let readMap H =
  [|for i in 1..H -> stdin.ReadLine()|]
  |> Array.map(fun line ->
    line.ToCharArray()
  )
  |> array2D

(* Main *)
open Library

let str = read string
let length = str.Length
let mutable count = 0
[1..length / 2]
|> List.iter(fun i ->
  if str.[i - 1] <> str.[length - i] then count <- count + 1
)

count |> printfn "%d"