input: Write a function to calculate the cube of a given integer. The function should take an integer as input and return its cube.
  assert cube_of_number(2)==8
  assert cube_of_number(3)==27
  assert cube_of_number(-1)==-1
  assert cube_of_number(0)==0
  assert cube_of_number(-2)==-8
output: def cube_of_number(n):
      return n ** 3