input: Write a function to calculate the factorial of a given number. If the given number is negative, return -1. If the number is zero, return 1.
  assert factorial(7)==5040
  assert factorial(0)==1
  assert factorial(-5)==-1
  assert factorial(1)==1
  assert factorial(4)==24
output: def factorial(n):
      if n < 0:
          return -1
      elif n == 0:
          return 1
      else:
          result = 1
          for i in range(1, n + 1):
              result *= i
          return result