input: Write a function to develop a Python program that clearly understands the concept of recursion. The function should define a recursive function to calculate the factorial of a given number and then use this function to calculate the factorial of 5. Print the result.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(0)==1
  assert factorial(3)==6
output: def factorial(n):
      if n == 0 or n == 1:
          return 1
      else:
          return n * factorial(n - 1)