input: Write a function to understand the concept of recursion by calculating the factorial of a given number using a recursive approach. If the given number is 1, return 1. Otherwise, return the product of the number and the factorial of the number minus one.
  assert factorial(5)==120
  assert factorial(1)==1
  assert factorial(0)==1
  assert factorial(3)==6
  assert factorial(-1)==1
output: def factorial(n):
      if n <= 1:
          return 1
      else:
          return n * factorial(n - 1)