input: Write a function to compute the cube of an integer input by the user and print the result.
  assert cube_of_number(2)==8
  assert cube_of_number(3)==27
  assert cube_of_number(-4)==-64
  assert cube_of_number(0)==0
  assert cube_of_number(1)==1
output: def cube_of_number(n):
      return n ** 3