input: Write a function to demonstrate the concept of recursion by calculating the factorial of a given number using a recursive function. The function should take an integer as input and return the factorial of that number. If the input number is 1, the function should return 1.
  
  def factorial(n):
     if n == 1:
        return 1
     else:
        return n * factorial(n - 1)
  
  assert factorial(5) == 120
  assert factorial(1) == 1
  assert factorial(3) == 6
  assert factorial(4) == 24
  assert factorial(0) == 1
output: def factorial(n):
     if n == 1 or n == 0:
        return 1
     else:
        return n * factorial(n - 1)