[<AutoOpen>]
module Program

open System

let read f = stdin.ReadLine() |> f
let reads f = stdin.ReadLine().Split() |> Array.map f

[<EntryPoint>]
let main _ =
    let S = read string |> Seq.toArray
    let revS = Array.rev S

    // eprintfn "%A" S
    // eprintfn "%A" revS

    Array.zip S revS
    |> Array.sumBy (fun (x, y) -> if x <> y then 1 else 0)
    |> fun x -> x / 2
    |> printfn "%i"
    0