$-1$ has three roots; Mathematica's choice (the "principal branch") happens to be complex. You'll want to use Surd[] for real-valued odd roots of negative numbers. See the linked thread for more details. – J. M.'s ennui – 2015-11-04T05:15:05.993

Excuse me! I read properly but not fluent in English to explain properly... – Dennis La Cotera – 2015-11-04T05:33:54.360

@J.M. thanks por your answer! – Dennis La Cotera – 2015-11-04T05:37:32.297

$-1$ has three roots;

Mathematica's choice (the "principal branch") happens to be complex. You'll want to use`Surd[]`

for real-valued odd roots of negative numbers. See the linked thread for more details. – J. M.'s ennui – 2015-11-04T05:15:05.993Excuse me! I read properly but not fluent in English to explain properly... – Dennis La Cotera – 2015-11-04T05:33:54.360

@J.M. thanks por your answer! – Dennis La Cotera – 2015-11-04T05:37:32.297