input: Write a function to define a recursive function that calculates the factorial of a given number and then calculates and prints the factorial of the number 5.
  def factorial(n):
     if n == 1:
        return 1
     else:
        return n * factorial(n - 1)
  
  result = factorial(5)
  print("Factorial of 5 is ", result)
  assert factorial(5)==120
  assert factorial(3)==6
  assert factorial(1)==1
  assert factorial(0)==None
output: def factorial(n):
     if n == 0:
        return None
     elif n == 1:
        return 1
     else:
        return n * factorial(n - 1)