// ievazquez
// // https://atcoder.jp/contests/abc147/tasks/abc147_b
// // implementation
//
import scala.io.StdIn._


object Main extends App {

  var ans = 0;
  val s = readLine
  val len = s.length
  for (i <- 0 until len / 2 ) {
    if(s(i) != s(len-i-1)) {
        ans += 1
    }
  }
  println(ans)
}