"use strict";

var fs = require('fs');

var main = function main(input) {
  var S = input.split('\n')[0].split('');
  var n = S.length;
  var max = Math.floor(n / 2);
  var hugCount = 0;

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

  console.log(hugCount);
};

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