input: Write a function to calculate the factorial of a given number. If the input number is less than 0, the function should return -1. If the input number is 0 or 1, the function should return 1.
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(1)==1
  assert factorial(4)==24
  assert factorial(-3)==-1
output: def factorial(n):
      if n < 0:
          return -1
      elif n == 0 or n == 1:
          return 1
      else:
          result = 1
          for i in range(2, n + 1):
              result *= i
          return result