let xs = stdin.ReadLine().ToCharArray()
let pivot = xs.Length / 2
let (s, s') = Array.splitAt pivot xs
let (rs, rs') = Array.splitAt (xs.Length%2) s'
Array.zip s (Array.rev rs')
|> Array.filter (fun (l, r) -> l <> r)
|> Array.length
|> printfn "%d"