fun main(args : Array<String>) {
    var S = readString()
    var chars = S.toCharArray()

    var reverseIndex = chars.size - 1
    var correctionCount = 0
    for (i in 0 until chars.size / 2){
        if(chars[i] != chars[reverseIndex]){
            correctionCount++
        }
        reverseIndex--
    }

    print(correctionCount)

}

fun readString() = readLine()!!