You can get the number of the level each level, if you can divide it by five, the next level loaded should be the bonus level, if not, then regular level.
If it can be divided you will get 1, 2, 3, 4 and so on. If it can't be divided, you'll get zero. So you only need one int compare (compare with 0, if equal, load regular level, if not equal, load bonus level).
