fun main(args :Array<String>) {
    val s = readLine()!!
    val rs = s.reversed()
    val last = (s.length - 1) / 2
    var ans = 0
    for (i in 0..last) {
        if (s[i] != rs[i]) {
            ans++
        }
    }
    println(ans)
}

fun readInteger() = readLine()!!.toInt()
fun readStringList() = readLine()!!.split(" ")
fun readIntegerList() = readStringList().map(String::toInt)
fun readLongList() = readStringList().map(String::toLong)
