From where is this statement? If it is a from a book it may well a mistraslation and or a typo, and I think it should be , otherwise the very statment of the result is basically non-sensical: how can -1 equal -1 to some power times the number of elements in ?? In the best of the cases this would mean , which of course is false.
Check this, I think it works fine with the correction.
Pd. Si estás estudiando de un libro traducido ten cuidado: son comunes los errores de traducción y los tipográficos.