function Main(input) {
  var count = 0;
  var i = 0;
  
  var string = input.split('');
  
  while (i < input.length / 2) {
    var last = input.length - i - 1;
  
    if (string[i] !== string[last]) {
      count++;
    }
    i++;
  }
  
  console.log(count);
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'));