object Main extends App {
  val target = io.StdIn.readLine()
  
  val (former, latter) = if (target.length % 2 == 0)
  	target.splitAt(target.length / 2)
  else
  	(target.substring(0, target.length / 2), target.substring(target.length / 2 + 1))
  
  val answer = former.zip(latter.reverse).count(pair => pair._1 != pair._2)
  
  println(answer)
}