import Foundation

func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}
func main() {
    let S = Array(readLine()!)
    var ans = 0
    for i in stride(from: 0, to: S.count/2, by: 1){
        if S[i] != S[S.count-i-1]{
            ans += 1
        }
    }
    print(ans)
}
main()