object Main extends scala.App {
    import java.util
    import scala.collection._
    import scala.collection.mutable
    import scala.math._

    val sc = new java.util.Scanner(System.in)
    val S = sc.next()

    val (pre, suf) = if (S.length % 2 == 0) {
        val n = S.length / 2
        (S.substring(0, n), S.substring(n).reverse)
    } else {
        val n = S.length / 2
        (S.substring(0, n), S.substring(n + 1).reverse)
    }

    val ans = (pre zip suf)
            .filter(p => p._1 != p._2)
            .foldLeft(0)((cnt, ch) => cnt + 1)
    println(ans)
}