function main(arg) {
  var arr = arg.split('')
  var len = arr.length

  var result = 0
  for (var i = 0; i < len; i++) {
    if (arr[i] !== arr[len - 1 - i]) {
      arr[i] = arr[len - 1 - i]
      result++
    }
  }

  console.log(result)
}

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