function Main(input) {
    var s_ = input;
    var s = [];
    var frag = 0;
    for(var j=0;j<s_.length;j++){
        s.push(s_[j]);
    }
    for(var i=0;i<s.length;i++){
        if(s[i] != s[s.length-1-i]){
            s[s.length-1-i] = s[i];
            frag++
        }
    }
    console.log(frag);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));