function Main (S) {
  S = S.split("\n")[0].split("");
  var re = [];
  var ans = 0;
  for (var i of S) {
    re.unshift(i);
  }
  for (var i = 0; i < S.length; i++) {
    if (S[i] != re[i]) {
      ans++;
    }
  }
  console.log(ans/2);
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));