input: Write a function to demonstrate the concept of recursion in Python by calculating the factorial of a given number using a recursive function. The function should return the factorial of the number. If the given number is less than 1, the function should return 1.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(3)==6
  assert factorial(0)==1
  assert factorial(-1)==1
output: def factorial(n):
      if n < 1:
          return 1
      else:
          return n * factorial(n - 1)