function Main(input) {
    const str = input.split("");
    let res = 0;
    for (let i = 0; i < Math.ceil(str.length / 2); i++) {
        if (str[i] !== str[str.length-1-i]) {
            res++;
        }
    }
    console.log(res);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
