import java.io.{BufferedReader, InputStreamReader}

object Main {
  def main(args: Array[String]): Unit = {
    val in = new BufferedReader(new InputStreamReader(System.in))
    val s = in.readLine()

    System.out.println(
      s.toCharArray.zip(s.toCharArray.reverse).count {
        case (a,b) => a != b
      } / 2
    )
  }
}
