input: Write a function to calculate the factorial of a given integer. If the integer is less than or equal to 1, return 1. Otherwise, recursively return the product of the integer and the factorial of the integer minus one. Then prompt the user to enter a number, and print out the factorial of that number.
  assert factorial(0) == 1
  assert factorial(1) == 1
  assert factorial(5) == 120
  assert factorial(7) == 5040
  assert factorial(10) == 3628800
output: def factorial(n):
      if n <= 1:
          return 1
      else:
          return n * factorial(n - 1)