object Main extends App {
    def main() = {
      val in = readLine()
      val arr = in.toArray
      val rev = arr.reverse
      val mid = arr.size / 2
      println(arr.take(mid).zip(rev.take(mid)).foldLeft(0)((a, b) => if (b._1 != b._2) a + 1 else 1))
    }
}
