import java.lang.Math

fun main(args: Array<String>)
{
    val s = readLine()!!.toCharArray()

    var cnt = 0
    for (i in 0..(s.size/2-1)) {
        val right = s.size - 1 - i
        if (s[i] != s[right]) cnt++
    }

    println(cnt)
}
