input: Write a python function to count number of cubes of size k in a cube of size n.
  assert No_of_cubes(2,1) == 8
  assert No_of_cubes(5,2) == 64
  assert No_of_cubes(1,1) == 1
  
output: def No_of_cubes(n, k):
      return (n - k + 1) ** 3