#![allow(non_snake_case)]
use proconio::{marker::Chars, *};

// https://maguro.dev/blog/debug-macro/
#[allow(unused_macros)]
macro_rules! debug {
    ($($a:expr),* $(,)*) => {
        #[cfg(debug_assertions)]
        eprintln!(concat!($("| ", stringify!($a), " = {:?} "),*, "|"), $(&$a),*);
    };
}

fn main() {
    std::thread::Builder::new()
        .stack_size(256 * 1024 * 1024)
        .spawn(solve)
        .unwrap()
        .join()
        .unwrap();
}

fn solve() {
    input! {
        s: Chars
    }

    let n = s.len();
    let mut res = 0;
    for i in 0..n / 2 {
        res += (s[i] != s[n - i - 1]) as usize;
    }
    println!("{res}");
}