>>> from potencies import potencies_2
>>> potencies_2 (2, 100, 6)
16
>>> potencies_2 (2, 100, 5)
-1
>>> potencies_2 (5, 100, 5)
5
>>> potencies_2 (5, 100, 3)
-1
>>> potencies_2 (7, 1000, 3)
343