input: Write a function to calculate the factorial of a given number using recursion. If the input number is less than 1, return 1.
  def factorial(n):
     if n < 1:
        return 1
     else:
        return n * factorial(n - 1)
  
  assert factorial(5)==120
  assert factorial(3)==6
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(-2)==1
output: def factorial(n):
     if n < 1:
        return 1
     else:
        return n * factorial(n - 1)