"use strict";

var main = function main(input) {
  var S = input.split('').map(function (n) {
    return n - 0;
  });
  var hugCount = 0;

  for (var i = 0; i < S.length; i++) {
    if (S[i] != S[S.length - i - 1]) hugCount++;
  }

  var answer = Math.floor(hugCount / 2);
  console.log(answer);
};

main(fs.readFileSync('/dev/stdin', 'utf8'));